Update the man page

[ruiu]

Summary

Add a description about the compression level of the debug info.

[MaskRay]

lld/docs/ld.lld.1

126

The default is 1.

342

1

[ruiu]

• address review comment

[MaskRay]

LGTM, but it would probably be wise to get a second pair of eyes on this change.

[grimar]

A question: do we need to specify the particular compression level in docs? My feeling is that
belongs to internal implementation, it is not really useful for user to know the level probably?

Imagine if we switch to another zlib implementation, like was suggested in the original PR,
what if it doesn't have these levels (I do not know if it has).

What would be probably better is to say something like:
"By default we use a compression, that is fast and effective enough, but if you want to
reduce the size of debug sections even more, you can specify -O2 to enable more aggressive,
but slower compression"

[ruiu]

In D71385#1781099, @grimar wrote:

A question: do we need to specify the particular compression level in docs? My feeling is that
belongs to internal implementation, it is not really useful for user to know the level probably?

Imagine if we switch to another zlib implementation, like was suggested in the original PR,
what if it doesn't have these levels (I do not know if it has).

What would be probably better is to say something like:
"By default we use a compression, that is fast and effective enough, but if you want to
reduce the size of debug sections even more, you can specify -O2 to enable more aggressive,
but slower compression"

I understand your point, but if I were to read your explanation for the first time, I would wonder what the actual compression levels are. People should be familiar with the notion of the compression level by gzip command option -0, ..., -9, and giving the numbers seems to be a most clear way to tell them what lld is doing. The numbers are not promise or anything, we can change that anytime, since they are only for documentation purpose.

[grimar]

Ok, LGTM

lld/docs/ld.lld.1

 .Fl -color-diagnostics Ns = Ns Cm auto .
 .It Fl -compress-debug-sections Ns = Ns Ar value
 Compress DWARF debug sections.
 .Ar value
 may be
 .Cm none
 or
 .Cm zlib .
+The default compression level is 0 (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
 Assign space to common symbols.
 .It Fl -defsym Ns = Ns Ar symbol Ns = Ns Ar expression
 Define a symbol alias.
 .Ar expression
 may be another symbol or a linker script expression.

 may be:
 .Pp
 .Bl -tag -width 2n -compact
 .It Cm 0
 Disable string merging.
 .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 0.
 .El
 .Pp
 .Fl O Ns Cm 1
 is the default.
 .It Fl -oformat Ns = Ns Ar format
 Specify the format for the output object file.
 The only supported
 .Ar format