Minor bug fixes.
- Changed 'bIgnoreDefRate' to 'bIgnoreDefRaceRate' in Pre-Renewal item database. (bugreport:8674)
http://rathena.org/board/tracker/issue-8674-item-bonus-error-or-not/
- Added missing 'bFixedCastrate' bonus for item 1832. (bugreport:8422)
http://rathena.org/board/tracker/issue-8422-vellum-claw-not-giving-insta-cast-on-10/
- Clarified 'getgmlevel()' and 'getgroupid()' functions in script documentation. (bugreport:8586, pull #46)
http://rathena.org/board/tracker/issue-8586-script-commandstxt-doc-getgroupid/
- Added Buying Store tables to source documentation.

Signed-off-by: Euphy <euphy.raliel@rathena.org>