[3.8] bpo-15817: gdbinit: Document commands after defining them (GH-15021) (#15744)

[3.8] bpo-15817: gdbinit: Document commands after defining them (GH-15021) (#15744)

The gdb manual[1] says the following for "document":

  The command commandname must already be defined.

[1] https://sourceware.org/gdb/current/onlinedocs/gdb/Define.html

And indeed when trying to use the gdbinit file with gdb 8.3, I get:

  .../cpython/Misc/gdbinit:17: Error in sourced command file:
  Undefined command: "pyo".  Try "help".

Fix this by moving all documentation blocks after the define blocks.

This was introduced in GH-6384.
(cherry picked from commit 1f86fdcfc57270ee569cc58269a4e08afe7608ec)

Authored-by: Florian Bruhin <me@the-compiler.org>