Update help text for commands implemented in Python

Update help text for commands implemented in Python

Philippe pointed out that the "enable frame-filter" help text looked
funny.

While fixing this I noticed a few more problems in the help text of
commands written in Python:

* Trailing newlines
* Wrong style for metasyntactic variables
* Indentation of the text
* ... and finally, I thought the function usage lines didn't need
  that extra newline -- someday I'd like to be able to write a
  "usage" command that just greps for the Usage line, so ideally it
  would be tighter than what was done here

This patch fixes all the problems I noticed.

gdb/ChangeLog
2018-10-06  Tom Tromey  <tom@tromey.com>

	* python/lib/gdb/function/strfns.py (_MemEq, _StrLen, _StrEq)
	(_RegEx): Reformat help text.
	* python/lib/gdb/function/caller_is.py (CallerIs, CallerMatches)
	(AnyCallerIs, AnyCallerMatches): Reformat help text.
	* python/lib/gdb/function/as_string.py (_AsString): Reformat help
	text.
	* python/lib/gdb/command/xmethods.py (InfoXMethod)
	(EnableXMethod, DisableXMethod): Remove help indentation.
	Capitalize meta-syntactic variables.
	* python/lib/gdb/command/unwinders.py (InfoUnwinder)
	(EnableUnwinder, DisableUnwinder): Remove help indentation.
	Capitalize meta-syntactic variables.
	* python/lib/gdb/command/explore.py (ExploreCommand)
	(ExploreValueCommand, ExploreTypeCommand): Reformat help text.
	* python/lib/gdb/command/type_printers.py (InfoTypePrinter)
	(EnableTypePrinter, DisableTypePrinter): Remove help indentation.
	* python/lib/gdb/command/pretty_printers.py (InfoPrettyPrinter):
	Remove help indentation.
	(EnablePrettyPrinter, DisablePrettyPrinter): Likewise.
	* python/lib/gdb/command/frame_filters.py (EnableFrameFilter)
	(DisableFrameFilter, SetFrameFilterPriority)
	(ShowFrameFilterPriority, InfoFrameFilter): Reword help text.