Tweak formatting and docs for pretty printers

Tweak formatting and docs for pretty printers

	* python/libstdcxx/v6/printers.py (UniquePointerPrinter.to_string):
	Remove redundant parentheses.
	(RbtreeIterator, StdRbtreeIteratorPrinter): Add docstrings.
	(StdForwardListPrinter.to_string): Remove redundant parentheses.
	(StdExpOptionalPrinter.to_string): Use string formatting instead of
	concatenation.
	(StdVariantPrinter.to_string, StdNodeHandlePrinter.to_string)
	(TemplateTypePrinter): Adjust whitespace.

From-SVN: r243689