runtime(termdebug): more termdebug fixes and improvements (#12892)

runtime(termdebug): more termdebug fixes and improvements (#12892)

- Fix and attempt to simplify :Frame/:Up/:Down documentation.

- Accept a count instead for :Up/:Down/+/-.

- Update the "Last Change" dates.

- Fix a missing :let (caused an error if gdb fails to start).

- Wipe the prompt buffer when ending prompt mode (if it exists and wasn't wiped
  by the user first). Avoids issues with stale prompt buffers (such as E95 when
  starting a new prompt mode session).

- Kill the gdb job if the prompt buffer is unloaded (similar to what's done for
  a terminal buffer). Fixes not being able to start a new termdebug session if
  the buffer was wiped by the user, for example.

Signed-off-by: Christian Brabandt <cb@256bit.org>