Add guidance for usage of description tags (e.g. [tag], #tag).

- Define what "tags" means in this context
- Note that usage of tags is optional
- Recommend that usage of tags is kept limited on the first line, in order to avoid obscuring the content

PiperOrigin-RevId: 530335206

review/developer/cl-descriptions.md

@@ -126,6 +126,53 @@ The first sentence describes what's actually being done. The rest of the
 description explains *why* the change is being made and gives the reviewer a lot
 of context.
 
+## Using tags {#tags}
+
+Tags are manually entered labels that can be used to categorize CLs. These may
+be supported by tools or just used by team convention.
+
+For example:
+
+-   "[tag]"
+-   "[a longer tag]"
+-   "#tag"
+-   "tag:"
+
+Using tags is optional.
+
+When adding tags, consider whether they should be in the [body](#informative) of
+the CL description or the [first line](#firstline). Limit the usage of tags in
+the first line, as this can obscure the content.
+
+Examples with and without tags:
+
+``` {.good}
+// Tags are okay in the first line if kept short.
+[banana] Peel the banana before eating.
+
+// Tags can be inlined in content.
+Peel the #banana before eating.
+
+// Tags are optional.
+Peel the banana before eating.
+
+// Multiple tags are acceptable if kept short.
+#banana #apple: Assemble a fruit basket.
+
+// Tags can go anywhere in the CL description.
+> Assemble a fruit basket.
+>
+> #banana #apple
+```
+
+``` {.bad}
+// Too many tags (or tags that are too long) overwhelm the first line.
+//
+// Instead, consider whether the tags can be moved into the description body
+// and/or shortened.
+[banana peeler factory factory][apple picking service] Assemble a fruit basket.
+```
+
 ## Generated CL descriptions
 
 Some CLs are generated by tools. Whenever possible, their descriptions should