(Targets): Document "set/show architecture". Remove

	(Targets): Document "set/show architecture".  Remove
	redundant index entry for "target" command.
	(Backtrace): Add index entries.
	(Symbols, Fortran): Document the "set case-sensitive" command.
	(DJGPP Native): Document "set com1base", "set com1irq", etc.
	(Print Settings): Add index entry for "set demangle-style".
	(Target Commands): Document "set download-write-size".
	(Debugging Output): Document "set exec-done-display".