docs: Update documentation for add_*_script

docs/markdown/Reference-manual.md

@@ -1735,6 +1735,8 @@ the following methods.
   0.49.0, the function only accepted a single argument. Since 0.54.0
   the `MESON_SOURCE_ROOT` and `MESON_BUILD_ROOT` environment variables
   are set when dist scripts are run.
+  *(Since 0.55.0)* The output of `configure_file`, `files`, and `find_program`
+  as well as strings.
 
 - `add_install_script(script_name, arg1, arg2, ...)` causes the script
   given as an argument to be run during the install step, this script
@@ -1742,6 +1744,9 @@ the following methods.
   `MESON_BUILD_ROOT`, `MESON_INSTALL_PREFIX`,
   `MESON_INSTALL_DESTDIR_PREFIX`, and `MESONINTROSPECT` set.
   All positional arguments are passed as parameters.
+  *(Since 0.55.0)* The output of `configure_file`, `files`, `find_program`,
+  `custom_target`, indexes of `custom_target`, `executable`, `library`, and
+  other built targets as well as strings.
 
   *(added 0.54)* If `meson install` is called with the `--quiet` option, the
   environment variable `MESON_INSTALL_QUIET` will be set.
@@ -1772,6 +1777,8 @@ the following methods.
   executable given as an argument after all project files have been
   generated. This script will have the environment variables
   `MESON_SOURCE_ROOT` and `MESON_BUILD_ROOT` set.
+  *(Since 0.55.0)* The output of `configure_file`, `files`, and `find_program`
+  as well as strings.
 
 - `backend()` *(added 0.37.0)* returns a string representing the
   current backend: `ninja`, `vs2010`, `vs2015`, `vs2017`, `vs2019`,

docs/markdown/snippets/add_foo_script_type_additions.md

@@ -0,0 +1,24 @@
+## meson.add_*_script methods accept new types
+
+All three (`add_install_script`, `add_dist_script`, and
+`add_postconf_script`) now accept ExternalPrograms (as returned by
+`find_program`), Files, and the output of `configure_file`. The dist and
+postconf methods cannot accept other types because of when they are run.
+While dist could, in theory, take other dependencies, it would require more
+extensive changes, particularly to the backend.
+
+```meson
+meson.add_install_script(find_program('foo'), files('bar'))
+meson.add_dist_script(find_program('foo'), files('bar'))
+meson.add_postconf_script(find_program('foo'), files('bar'))
+```
+
+The install script variant is also able to accept custom_targets,
+custom_target indexes, and build targets (executables, libraries), and can
+use built executables a the script to run
+
+```meson
+installer = executable('installer', ...)
+meson.add_install_script(installer, ...)
+meson.add_install_script('foo.py', installer)
+```