gh-90539: doc: Expand on what should not go into CFLAGS, LDFLAGS

Doc/using/configure.rst

@@ -747,6 +747,17 @@ Compiler flags
    extensions.  Use it when a compiler flag should *not* be part of the
    distutils :envvar:`CFLAGS` once Python is installed (:issue:`21121`).
 
+   In particular, :envvar:`CFLAGS` should not contain:
+
+   - the compiler flag `-I` (for setting the search path for include files);
+     the `-I` flags are processed from left to right, and any flags in
+     :envvar:`CFLAGS` would take precedence over user- and package-supplied `-I`
+     flags.
+
+   - hardening flags such as `-Werror` because distributions cannot control
+     whether packages installed by users conform to such heightened
+     standards.
+
    .. versionadded:: 3.5
 
 .. envvar:: EXTRA_CFLAGS
@@ -859,6 +870,13 @@ Linker flags
    :envvar:`CFLAGS_NODIST`.  Use it when a linker flag should *not* be part of
    the distutils :envvar:`LDFLAGS` once Python is installed (:issue:`35257`).
 
+   In particular, :envvar:`LDFLAGS` should not contain:
+
+   - the compiler flag `-L` (for setting the search path for libraries);
+     the `-L` flags are processed from left to right, and any flags in
+     :envvar:`CFLAGS` would take precedence over user- and package-supplied `-L`
+     flags.
+
 .. envvar:: CONFIGURE_LDFLAGS_NODIST
 
    Value of :envvar:`LDFLAGS_NODIST` variable passed to the ``./configure``

Misc/NEWS.d/next/Documentation/2022-05-12-23-47-17.gh-issue-90539.nmRgOZ.rst

@@ -0,0 +1 @@
+Expand the documentation of the configure variables `CFLAGS_NODIST`, `LDFLAGS_NODIST`.