Document lazy computation for pretty-printer "children" method

Document lazy computation for pretty-printer "children" method

I found out recently that some users didn't know that the Python
pretty-printers "children" method should compute its result lazily.
This has been a good idea since the earliest days, but wasn't
mentioned in the docs.  This patch adds some text to this effect.

gdb/doc/ChangeLog
2019-05-10  Tom Tromey  <tromey@adacore.com>

	* python.texi (Pretty Printing API): Mention lazy computation for
	"children".