docs: Update debugging docs to mention Meson build options

The behaviour of the Meson build has changed a little vs what we did in
autotools. In autotools, --enable-debug was a tristate (yes, no,
undefined), with all three options resulting in different macro
definitions.

In Meson, we have a bistate of --buildtype={debug,debugoptimized} vs
--buildtype=(anything else). There is no way to automatically define
G_DISABLE_ASSERT or G_DISABLE_CHECKS while building GLib — you need to
define them in your CPPFLAGS in your environment instead.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

docs/debugging.txt

@@ -4,7 +4,7 @@ Traps (G_BREAKPOINT) and traces for the debuging
 
 Some code portions contain trap variables that can be set during
 debugging time if G_ENABLE_DEBUG has been defined upon compilation
-(use the --enable-debug=yes option to configure for this, macros.txt
+(use the --buildtype=debug option to configure for this, macros.txt
 covers more details).
 Such traps lead to immediate code halts to examine the current
 program state and backtrace.

docs/macros.txt

@@ -3,13 +3,14 @@
 GLib's configure options and corresponding macros
 =================================================
 
---enable-debug=no
-	-DG_DISABLE_ASSERT -DG_DISABLE_CHECKS
---enable-debug=minimum	[default for stable branches]
+--buildtype={plain,release,minsize,custom}
 	none
---enable-debug=yes	[default for development branches]
+--buildtype={debug,debugoptimized}	[debugoptimized is the default]
 	-DG_ENABLE_DEBUG -g
 
+Available to define yourself:
+	-DG_DISABLE_ASSERT -DG_DISABLE_CHECKS
+
 Besides these, there are some local feature specific options, but my main
 focus here is to concentrate on macros that affect overall GLib behaviour
 and/or third party code.
@@ -22,8 +23,8 @@ G_DISABLE_ASSERT
 	The g_assert() and g_assert_not_reached() become non-functional
 	with this define. The motivation is to speed up end-user apps by
 	avoiding expensive checks.
-	This macro can affect third-party code. --enable-debug=no will only
-	disable the assertion macros for GLib itself, but third-party code
+	This macro can affect third-party code. Defining it when building GLib
+	will only disable the assertion macros for GLib itself, but third-party code
 	that passes -DG_DISABLE_ASSERT to the compiler upon its own build
 	will end up with the non-functional variants after including glib.h
 	as well.