gdb, doc: Add ieee_half and bfloat16 to list of predefined target types.

For some reason these two weren't added to the list when they were orginally
added to GDB.

gdb/doc/ChangeLog:
2021-09-21  Felix Willgerodt  <felix.willgerodt@intel.com>

	* gdb.texinfo (Predefined Target Types): Mention ieee_half and bfloat16.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 44837d9..3eafcfb 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -45972,12 +45972,18 @@
 address.  The stack pointer and any dedicated address registers
 may be marked as data pointers.
 
+@item ieee_half
+Half precision IEEE floating point.
+
 @item ieee_single
 Single precision IEEE floating point.
 
 @item ieee_double
 Double precision IEEE floating point.
 
+@item bfloat16
+The 16-bit @dfn{brain floating point} format used e.g.@: by x86 and ARM.
+
 @item arm_fpa_ext
 The 12-byte extended precision format used by ARM FPA registers.