[docs] clarify todo about deinitializing a single element

[glessard]

Clarify TODO item about deinitializing a single element in UnsafeMutablePointer.deinitialize(count:).

[glessard]

@atrick This tweak has been hinted at as a FIXME in this function for years. Obviously it's fine when deinitializing a number of elements that's a constant in source code. Would this be a bad thing when the argument is some statically unknown variable?

[eeckstein]

Yes, the TODO meant something different: if it's statically a constant of 1, replace it with a simple destroy. In not done already, that can be easily done in IRGen.
Such a check at runtime adds code size. We should only do this if there is evidence (e.g. from benchmarks) that this really gives significant performance wins.

[glessard]

@eeckstein I made it a clearer TODO instead of a FIXME.

[eeckstein]

thanks

[glessard]

@swift-ci please smoke test and merge