It took me a long time to debug why my configs were not being loaded,
and judging from online discussions I'm not the only one. Make the
comment in the default my.cnf a bit more helpful.
The !includedir directive is implemented in mysys/my_default.c.
- f_extensions[] is a list of file extensions. It includes .ini and .cnf
on Windows, and only .cnf on all other platforms.
- search_default_file_with_ext() contains the !includedir directive. It
filters files in the directory to those matching f_extensions[].
This file should only be applicable on Unix-like platforms, so only
files with the .cnf extension are read.
Closes#1485