bug#24923: 25.1; Lisp watchpoints

[npostavs]

Eli Zaretskii <eliz@gnu.org> writes:
>
> Our coding conventions put the logical operators at the beginning of a
> line, not at EOL.

Fixed this, and added documentation.  Also, watcher functions are now
listed in describe-variable output.

Does it make sense to mention the use of the `watchers' symbol property
in the manual?  Since I've added a `get-variable-watchers' it's now
possible to ignore the symbol property as an implementation detail.

v6-0001-Add-lisp-watchpoints.patch
Description: patch

v6-0002-Show-watchpoints-when-describing-variables.patch
Description: patch

v6-0003-Add-function-to-trigger-debugger-on-variable-writ.patch
Description: patch

v6-0004-Ensure-redisplay-using-variable-watcher.patch
Description: patch

v6-0005-Add-tests-for-watchpoints.patch
Description: patch

v6-0006-Document-watchpoints.patch
Description: patch