docs: update description of supported functions for feature options

We support these for a couple more things now.

docs/markdown/Build-options.md

@@ -76,10 +76,14 @@ This type is available since version 0.44.0
 A `feature` option has three states: `enabled`, `disabled` or `auto`.
 It is intended to be passed as value for the `required` keyword
 argument of most functions. Currently supported in
-[[dependency]],
+[[add_languages]],
 [[compiler.find_library]],
-[[find_program]] and
+[[compiler.has_header]],
-[[add_languages]] functions.
+[[dependency]],
+[[find_program]],
+[[import]] and
+[[subproject]]
+functions.
 
 - `enabled` is the same as passing `required : true`.
 - `auto` is the same as passing `required : false`.