[DOC] Undocumented entries in IRB.conf

[BurdetteLamar]

The recent revision of the main IRB documentation has three :stopdoc:/:startdoc: directive pairs that try to mention but omit several undocumented entries in IRB.conf:

• IRB.conf[:LC_MESSAGES]
• IRB.conf[:MEASURE] IRB.conf[:MEASURE_CALLBACKS] IRB.conf[:MEASURE_PROC]
• IRB.conf[:CONTEXT_MODE]

The directives did not work, however, so the mentions have leaked into the finished html doc,

I'll be removing those from the main doc, and so am mentioning them here to prevent loss.