Auto merge of #12705 - Turbo87:patch-1, r=epage

doc/reference/manifest: Adjust `keywords` description

This adjusts the naming rules for keywords to match the implemented reality:

https://github.com/rust-lang/crates.io/blob/aab95692baa0dd2374a2ab5cb2cb2d89d7b2a2eb/src/models/keyword.rs#L56-L64

see also:

- rust-lang/rfcs#3488 (comment)
- rust-lang/rfcs#3488 (comment)

src/doc/src/reference/manifest.md

@@ -306,9 +306,9 @@ words that would help someone find this crate.
 keywords = ["gamedev", "graphics"]
 ```
 
-> **Note**: [crates.io] has a maximum of 5 keywords. Each keyword must be
-> ASCII text, start with a letter, and only contain letters, numbers, `_` or
-> `-`, and have at most 20 characters.
+> **Note**: [crates.io] allows a maximum of 5 keywords. Each keyword must be
+> ASCII text, have at most 20 characters, start with an alphanumeric character,
+> and only contain letters, numbers, `_`, `-` or `+`.
 
 ### The `categories` field