Document which characters are allowed for pre-/post-fixes of OWNERS files

Change-Id: I92b0a371c2ef0af85f36ddd26cd3b7aa443b8911
Signed-off-by: Edwin Kempin <ekempin@google.com>
Reviewed-on: https://gerrit-review.googlesource.com/c/plugins/code-owners/+/409280
Reviewed-by: Nitzan Gur-Furman <nitzan@google.com>
Tested-by: Zuul <zuul-63@gerritcodereview-ci.iam.gserviceaccount.com>

resources/Documentation/backends.md

diff --git a/resources/Documentation/backends.md b/resources/Documentation/backends.md
index 7069717..63ff69a 100644
--- a/resources/Documentation/backends.md
+++ b/resources/Documentation/backends.md
@@ -43,6 +43,9 @@
 | `find-owners` | `<prefix>_OWNERS`, `OWNERS_<extension>`                   |
 | `proto`       | `<prefix>_OWNERS_METADATA`, `OWNERS_METADATA_<extension>` |
 
+For "`<prefix>`" and "`<extension>`" only strings are allowed that match with
+"`([A-Za-z0-9_-])+`".
+
 Primary and secondary code owner config files are [validated](validation.html)
 by the `@PLUGIN@` plugin when they are changed to ensure that they are always
 parsable and valid.