Update reference manual and release notes

docs/markdown/Reference-manual.md

@@ -127,6 +127,7 @@ These are all the supported keyword arguments:
 - `output` the output file name (since v0.41.0, 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
+- `capture` when this argument is set to true, Meson captures `stdout` of the `command` and writes it to the target file specified as `output`. Available since v0.41.0.
 - `install_dir` the subdirectory to install the generated file to (e.g. `share/myproject`), if omitted the file is not installed.
 
 ### custom_target()

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

@@ -80,3 +80,9 @@ but actually isn't. For example when:
 - `build_machine` is macOS and `host_machine` is the iOS Simulator
 - the `build_machine`'s libc is glibc but the `host_machine` libc is uClibc
 - code relies on kernel features not available on the `build_machine`
+
+## Support for capturing stdout of a command in configure_file
+
+`configure_file()` now supports a new keyword - `capture`. When this argument
+is set to true, Meson captures `stdout` of the `command` and writes it to
+the target file specified as `output`.