Added documentation.

docs/markdown/Reference-manual.md

@@ -1385,6 +1385,12 @@ the following methods.
   /path/to/meson.py introspect`. The user is responsible for splitting
   the string to an array if needed.
 
+- `override_find_program(progname, program)` specifies that whenever
+  `find_program` is used to find a program named `progname`, Meson
+  should not not look it up on the system but instead return
+  `program`, which may either be the result of `find_program` or
+  `configure_file`.
+
 - `project_version()` returns the version string specified in `project` function call.
 
 - `project_license()` returns the array of licenses specified in `project` function call.

docs/markdown/snippets/find-override.md

@@ -0,0 +1,27 @@
+## Can override find_program
+
+It is now possible to override the result of `find_program` to point
+to a custom program you want. The overriding is global and applies to
+every subproject from there on. Here is how you would use it.
+
+In master project
+
+```meson
+subproject('mydep')
+```
+
+In the called subproject:
+
+```meson
+prog = find_program('my_custom_script')
+meson.override_find_program('mycodegen', prog)
+```
+
+In master project (or, in fact, any subproject):
+
+```meson
+genprog = find_program('mycodegen')
+```
+
+Now `genprog` points to the custom script. If the dependency had come
+from the system, then it would point to the system version.