[compiler-rt] Update comments in asan header to doxygen format; NFC.

Add description to undocumented functions.
Improve spelling, grammar and formatting.

Patch by: Craig Flores, Filipe Cabecinhas, Pierre Gousseau

Reviewed By: Johannes Doerfert

Differential Revision: https://reviews.llvm.org/D62245

llvm-svn: 363038