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.

gdb/python/python-internal.h

@@ -136,6 +136,8 @@ PyObject *gdbpy_history (PyObject *self, PyObject *args);
 PyObject *gdbpy_breakpoints (PyObject *, PyObject *);
 PyObject *gdbpy_frame_stop_reason_string (PyObject *, PyObject *);
 PyObject *gdbpy_lookup_symbol (PyObject *self, PyObject *args, PyObject *kw);
+PyObject *gdbpy_lookup_global_symbol (PyObject *self, PyObject *args,
+				      PyObject *kw);
 PyObject *gdbpy_newest_frame (PyObject *self, PyObject *args);
 PyObject *gdbpy_selected_frame (PyObject *self, PyObject *args);
 PyObject *gdbpy_block_for_pc (PyObject *self, PyObject *args);