gdb/linux-nat.c: Add "Accessing inferior memory" section

This commit adds a new "Accessing inferior memory" comment section to
gdb/linux-nat.c that explains why we prefer /proc/pid/mem over
alternatives.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30453

Change-Id: I575b21ed697a85f3ff4c0ec58c04812db5005b76