Add gdb.lookup_global_symbol python function.

	Add gdb.lookup_global_symbol python function.
	* NEWS: Add entry.
	* python/py-symbol.c (gdbpy_lookup_global_symbol): New function.
	* python/python-internal.h (gdbpy_lookup_global_symbol): Declare it.
	* python/python.c (GdbMethods): Add entry for lookup_global_symbol.

	doc/
	* gdb.texinfo (Symbols In Python): Document lookup_global_symbol.
	Clarify behaviour of lookup_symbol when `block' argument is omitted,
	add description of result, fix @defun formatting.

	testsuite/
	* gdb.python/py-symbol.exp: Test lookup_global_symbol.