debuginfod-find.1: note on how to find a comp_dir

debuginfod-find.1: note on how to find a comp_dir

doc/ChangeLog:

* debuginfod-find.1: add a note to DESCRIPTION section for the 'source'
  subcommand, clarifying where to find the CU compilation-directory.

I'm looking at how to improve and document the workflow for using
debuginfod-find to study the behaviour of packaged software on a system, e.g.
in conjunction with a tracing tool like SystemTap. You can read the exact
compiled source code with debuginfod-find source and use that to figure out
which source code locations are interesting to trace. But that process has
several non-obvious or inconvenient steps.

This patch is a clarification to the debuginfod-find source man page pointing
out the eu-readelf command that can show the comp_dir in downloaded debuginfo.
Since debuginfod-find source could be picked up and used by a developer
unfamiliar with DWARF terminology, I think such a clarification saves time
for newbies figuring out what a CU compilation-directory is.

Signed-off-by: Serhei Makarov <serhei@serhei.io>