[ThinLTO][C-API] Correct api comments

Negative values never disabled the pruning - they simply set high values for the pruning interval.

The behaviour now is that negative values set the maximum pruning interval (which appears to have been the intention from the start) see https://reviews.llvm.org/D41231.

I have adjusted the comments to reflect this, removed any inaccurate statements, and corrected any typos I spotted in the English.

Differential Revision: https://reviews.llvm.org/D41279

llvm-svn: 321078