[ticket/7262] Backport set_config() and set_config_count() docs from develop.

PHPBB3-7262
[ticket/7262] Add $is_dynamic example to set_config() and set_config_count().

The logic is the other way around here in comparison to develop's
phpbb_config_db class, so add examples to make things more clear.

PHPBB3-7262
[ticket/7262] Add note about set_config() not updating is_dynamic.

PHPBB3-7262
Merge remote-tracking branch 'github-bantu/ticket/7262' into develop-olympus

* github-bantu/ticket/7262:
  [ticket/7262] Add note about set_config() not updating is_dynamic.
  [ticket/7262] Add $is_dynamic example to set_config() and set_config_count().
  [ticket/7262] Backport set_config() and set_config_count() docs from develop.