diff --git a/doc/docs.md b/doc/docs.md
index 26cadbf487..6b82e1c7cd 100644
--- a/doc/docs.md
+++ b/doc/docs.md
@@ -3312,6 +3312,8 @@ If a file has an environment-specific suffix, it will only be compiled for that
 - `.c.v` => will be used only by the C backend. These files can contain C. code.
 - `.x64.v` => will be used only by V's x64 backend.
 - `_nix.c.v` => will be used only on Unix systems (non Windows).
+- `_d_customflag.v` => will be used only if you pass `-d customflag` to V.
+That corresponds to `$if customflag ? {}`, but for a whole file, not just a single block.
 - `_${os}.c.v` => will be used only on the specific `os` system.
 For example, `_windows.c.v` will be used only when compiling on Windows, or with `-os windows`.
 - `_default.c.v` => will be used only if there is NOT a more specific platform file.