bitbake: documentation: bitbake: add file-checksums to varflags section

Fixes [YOCTO #11605] by:

- Adding definition of file-checksums to Variable Flags section.
- Describe data to add to list which adds external file dependencies.
- Write example on usage to prepend a value to file-checksums list.

(Bitbake rev: 6bc65e6402a74c9a65e29333a7e0f1f85dcbcf58)

Signed-off-by: Richard Elberger <rich@richelberger.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.rst

@@ -1496,6 +1496,23 @@ functionality of the task:
    directory listed is used as the current working directory for the
    task.
 
+- ``[file-checksums]``: Controls the file dependencies for a task. The
+  baseline file list is the set of files associated with
+  :term:`SRC_URI`. May be used to set additional dependencies on
+  files not associated with :term:`SRC_URI`.
+
+  The value set to the list is a file-boolean pair where the first
+  value is the file name and the second is whether or not it
+  physically exists on the filesystem. ::
+
+    do_configure[file-checksums] += "${MY_DIRPATH}/my-file.txt:True"
+
+  It is important to record any paths which the task looked at and
+  which didn't exist. This means that if these do exist at a later
+  time, the task can be rerun with the new additional files. The
+  "exists" True or False value after the path allows this to be
+  handled.
+
 -  ``[lockfiles]``: Specifies one or more lockfiles to lock while the
    task executes. Only one task may hold a lockfile, and any task that
    attempts to lock an already locked file will block until the lock is