Update reference manual and release notes

docs/markdown/Reference-manual.md

@@ -124,7 +124,7 @@ When a list of strings is passed to the `command:` keyword argument, it takes an
 These are all the supported keyword arguments:
 
 - `input` the input file name. If it's not specified in configuration mode, all the variables in the `configuration:` object (see above) are written to the `output:` file.
-- `output` the output file name. In configuration mode, the permissions of the input file (if it is specified) are copied to the output file.
+- `output` the output file name (may contain `@PLAINNAME@` or `@BASENAME@` substitutions). In configuration mode, the permissions of the input file (if it is specified) are copied to the output file.
 - `configuration` as explained above, this is where you pass the configuration data object as returned by `configuration_data()`
 - `command` as explained above, if specified, Meson does not create the file itself but rather runs the specified command, which allows you to do fully custom file generation
 - `install_dir` the subdirectory to install the generated file to (e.g. `share/myproject`), if omitted the file is not installed.

docs/markdown/Release-notes-for-0.41.0.md

@@ -67,3 +67,7 @@ coverage must be combined before producing a report (`coverage3 combine`.)
 All known issues have been fixed and Meson can now build reproducible Debian
 packages out of the box.
 
+## Extended template substitution in configure_file
+
+The output argument of `configure_file()` is parsed for @BASENAME@ and
+@PLAINNAME@ substitutions.