help: add additional information in the merge-tools section of config help

The merge tool configuration in general has always been a confusing topic for
user. We add mention of:
- possible pre-existing configuration,
- way to look at it,
- the more detailed help topic about how merge-tools configuration works.

This should help users find they way.

mercurial/help/config.txt

@@ -807,7 +807,9 b' Example::'
 ---------------
 
 This section configures external merge tools to use for file-level
-merges.
+merges. This section has likely been preconfigured at install time.
+Use :hg:`config merge-tools` to check the existing configuration.
+Also see :hg:`help merge-tools` for more details.
 
 Example ``~/.hgrc``::