Add documentation on checked_by license parameter (#41959)

This patch adds documentation on the checked_by license direcitve
parameter that was added in #40755 but was not documented at that time.

lib/spack/docs/packaging_guide.rst

@@ -6979,3 +6979,18 @@ you probably care most about are:
 You may also care about `license exceptions
 <https://spdx.org/licenses/exceptions-index.html>`_ that use the ``WITH`` operator,
 e.g. ``Apache-2.0 WITH LLVM-exception``.
+
+Many of the licenses that are currently in the spack repositories have been
+automatically determined. While this is great for bulk adding license
+information and is most likely correct, there are sometimes edge cases that
+require manual intervention. To determine which licenses are validated and
+which are not, there is the `checked_by` parameter in the license directive:
+
+.. code-block:: python
+
+   license("<license>", when="<when>", checked_by="<github username>")
+
+When you have validated a github license, either when doing so explicitly or
+as part of packaging a new package, please set the `checked_by` parameter
+to your Github username to signal that the license has been manually
+verified.