[gtk/matthiasc/for-master] docs: Add details about plain builds

[Matthias Clasen <matthiasc src gnome org>]

commit 714e5c72f6782191937c94a7cb7b6b750bd1ff5b
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Nov 17 12:21:29 2020 -0500

    docs: Add details about plain builds
    
    Mention that build type 'plain' puts the responsibility
    for debug features into the hands of the builder.
    
    Related: !2831

 docs/reference/gtk/building.md | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/docs/reference/gtk/building.md b/docs/reference/gtk/building.md
index 2ee660fb05..51c8802168 100644
--- a/docs/reference/gtk/building.md
+++ b/docs/reference/gtk/building.md
@@ -107,7 +107,9 @@ The `plain` build type provided by Meson should only be used when
 packaging GTK, and it's expected that packagers will provide their
 own compiler flags when building GTK. See the previous section for
 the list of environment variables to be used to define compiler and
-linker flags.
+linker flags. Note that with the plain build type, you are also
+responsible for controlling the debugging features of GTK with
+`-DG_ENABLE_DEBUG` and `-DG_DISABLE_CAST_CHECKS`.
 
 ## Dependencies {#dependencies}