Document hotdoc.generate_doc(depends:) kwarg

Document the results of commit bcc127b3f,
which added 'depends' and deprecated using
'dependencies' for custom targets.

Fixes: #11479

docs/markdown/Hotdoc-module.md

@@ -33,12 +33,19 @@ Generates documentation using [hotdoc] and installs it into `$prefix/share/doc/h
 
 * `sitemap` ([[@str]] or [[@file]]) (**required**): The hotdoc sitemap file
 * `index` ([[@str]] or [[@file]]) (**required**): Location of the index file
-* `dependencies`([[@build_tgt]]): Targets on which the documentation generation depends on.
+* `dependencies`([[@build_tgt]]): Build targets to use when generating documentation.
+* `depends`([[@custom_tgt]]): Custom targets on which this documentation target depends.
 * `subprojects`: A list of `HotdocTarget` that are used as subprojects for hotdoc to generate
   the documentation.
 * ... Any argument of `hotdoc` can be used replacing dashes (`-`) with underscores (`_`).
   For a full list of available parameters, just have a look at `hotdoc help`.
 
+*Changed in 0.64.1:* `depends:` added.
+Previously, all types of targets were accepted by `dependencies:`.
+This is now deprecated.
+Use `dependencies:` only with build targets, to pass their configuration to hotdoc.
+Use `depends:` to set up dependency relationships on custom targets.
+
 **Returns:**
 
 `HotdocTarget`: A [[custom_target]] with the
@@ -72,4 +79,4 @@ hotdoc.generate_doc('foobar',
 )
 ```
 
-[hotdoc]: https://hotdoc.github.io/
+[hotdoc]: https://hotdoc.github.io/