Cleanup of documentation for SoundEffectInstance. Removed unnecessary hold on the parent SoundEffect reference. Removed a duplicate initialization of the source buffer. Changed the use of Math.exp in the pitch conversion code to use Math.pow so that it can be understood better.
Merge pull request #1390 from totallyeviljake/ISSUE_1386_R1

Cleanup of documentation for SoundEffectInstance and misc code cleanup.
Merge branch 'develop' of https://github.com/mono/MonoGame into develop