doc: add unrepresented flags docs for configure

Added documentation to support usage of advanced features of
`configure` on Windows.

PR-URL: #28069
Fixes: #17331
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

doc/guides/maintaining-the-build-files.md

@@ -34,6 +34,14 @@ update the build and testing process on Windows.
 
 ## configure
 
+The `configure` script recognizes many CLI flags for special build formulas.
+Many are not represented by `vcbuild` shortcuts, and need to be passed
+either by:
+
+* Calling `python configure --XXX --YYY=PPPP` directly, followed by `vcbuild
+  noprojgen`
+* Setting `set config_flags=--XXX --YYY=PPPP` before calling `vcbuild`
+
 To see the help text, run `python configure --help`. Update this file when you
 need to update the configuration process.