bug#13775: 24.3.50; Omissions in documentation for crash reporting

[Dmitry Gutov <dgutov@yandex.ru>]

As a non-GDB-wielding user with not much C experience, I had a hard time
following the instructions in `report-emacs-bug' and the ones that
followed.

1. Calling `xbacktrace' requires src/.gdbinit to be loaded. It
a) requires the user to run gdb exactly from src/ (not `gdb src/emacs'),
b) requires them to modify the `auto-load safe-path', or that .gdbinit
is ignored.

2. "Compile without optimizations" - how do I do that? `configure
--help' doesn't seem to show any pertinent options. ...but wait, it says
I can override the choices made by the script.
a) Do I set the variable when calling `make', or do I have to re-run
./configure? Not obvious, the answer is "the latter".
b) I don't know the choice the script made, how do I not break
anything by overriding it? My first choice was "-O0", and that resulted
in an unoptimized build with no debugging symbols (I think).

3. In #13749 (which caused me to write this), Paul also suggests using
-DENABLE_CHECKING. If I'm not mistaken, this variable isn't documented
anywhere.