Tendonosis is one of the long-term diseases under the categorisation of RSI, the others inculding entrapment conditions (e.g carpal tunnel syndrome). Short-term repetitive strain conditions include tendonitis and tenosynovitis - these short-term conditions typically resolve within a few days to 6 weeks and are inflammation based [source], however entrapment conditions and tendonosis take considerably longer to resolve.

Causes

The tendinosis cycle begins when breakdown exceeds repair. Repetitive motion causes microinjuries that accumulate with time. Collagen breaks down and the tendon tries to repair itself, but the cells produce new collagen with an abnormal structure and composition.

The new collagen has an abnormally high Type III/Type I ratio. Experiments show that the excess Type III collagen at the expense of Type I collagen weakens the tendon, making it prone to further injury. Part of the problem is that the new collagen fibers are less organized into the normal parallel structure, making the tendon less able to withstand tensile stress along the direction of the tendon.

-- http://www.tendonpain.org/injury.shtml

The appearance of the tendon shifts from a reflective, “white, glistening and firm” surface to a “dull-appearing, slightly brown and soft” surface (mucoid degeneration)(2,3).

These changes result in a loss of strength in the tendon and increase the bulk of the tendon, both of which contribute to the cycle of injury and can set the stage for secondary conditions, such as tendinitis and nerve impingement. My study leads me to believe that, in the forearm and wrist, tendinosis can result in secondary carpal tunnel syndrome; this is because the thickening of the tendons with excess ground substance and the swelling of the surrounding tissue crowds and compresses the median nerve.

-- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312643/

The source of pain from tendinosis/tendinopathy is controversial. At first, doctors labeled chronic tendon injuries as "tendinitis" and attributed the pain to inflammation. Later, researchers observed that inflammatory cells were rarely seen in microscope slides of chronic tendon injuries, so the terms tendinosis and tendinopathy became more popular because they do not imply an inflammatory process.
.....
The pain from tendinosis probably comes partly from the physical injury itself (separation of collagen fibers and mechanical disruption of tissue) and partly from irritating biochemical substances that are produced as part of the injury process. The biochemical substances could irritate the pain receptors in the tendon and surrounding area.

-- http://www.tendonpain.org/injury.shtml

Treatment

The primary treatment goals for tendinosis are to: break the cycle of injury; reduce ground substance, pathologic vascularization, and subsequent tendon thickening; and optimize collagen production and maturation so that the tendon regains normal tensile strength(3).

-- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312643/

The general consensus is that rest, stretching, strengthening excersies and icing are the core recommendations for tendonosis, details and more can be found here at the bottom of the article. However, there is a interesting focus on recommending eccentric excersies.

Eccentric strengthening effectively stimulates collagen production, improves collagen alignment, and stimulates collagen cross-linkage formation, in turn improving tensile strength(5). 

-- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312643/

Eccentric strengthening exercise programs have been advocated as effective treatments for tendon overuse injuries and prevention of reinjury100-108. Arampatzis et al. demonstrated that exercises involving high tendon strain (mean and standard deviation, 4.72% ± 1.08%) were more effective than those producing low tendon strain (mean, 2.97% ± 0.47%) in triggering an adaptive response in human Achilles tendon100. Stanish et al. suggested that eccentric exercises prepare patients for return to functional, sports-related activities better than those that emphasize concentric muscle strengthening108. Multiple studies have demonstrated eccentric exercise regimens to be effective for the treatment of tendinopathy103,109-115. Ohberg et al. found decreased Achilles tendon thickness and normal tendon structure in response to a twelve-week eccentric training program in patients with chronic Achilles tendinosis103.

-- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3748997/