[docs][PATCH] doc: enhance SRCREV_FORMAT description

[Patrick Vogelaar <patrick.vogelaar.dev@mailbox.org>]

Adding documentation for having an empty first component for the
SRCREV_FORMAT.

Signed-off-by: Patrick Vogelaar <patrick.vogelaar.dev@mailbox.org>
---
 .../bitbake-user-manual-ref-variables.rst                   | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.rst b/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.rst
index 810f88689..07e754096 100644
--- a/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.rst
+++ b/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.rst
@@ -1670,6 +1670,12 @@ overview of their function and contents.
       ``AUTOINC`` placeholder is added and if needed. And, this placeholder
       is placed at the start of the returned string.
 
+      The :term:`SRCREV_FORMAT` can also take the form "_component2".
+      This assumes that there is a component in the :term:`SRC_URI` that does not
+      have a name assigned. While this is not considered good practice, it can be
+      usefull if a ``.bbappend`` file needs to extend the :term:`SRC_URI` with
+      an additional repository.
+
    :term:`STAMP`
       Specifies the base path used to create recipe stamp files. The path
       to an actual stamp file is constructed by evaluating this string and