Add a description about the compression level of the debug info.
Differential Revision: https://reviews.llvm.org/D71385
.Cm none
or
.Cm zlib .
+The default compression level is 1 (fastest) as the debug info usually
+compresses well at that level, but if you want to compress it more,
+you can specify
+.Fl O2
+to set the compression level to 6.
.It Fl -cref
Output cross reference table.
.It Fl -define-common , Fl d
.It Cm 1
Enable string merging.
.It Cm 2
-Enable string tail merging.
+Enable string tail merging. If
+.Fl -compress-debug-sections
+is given, compress debug sections at compression level 6 instead of 1.
.El
.Pp
.Fl O Ns Cm 1