Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

content/appendices.md

@@ -612,7 +612,7 @@ users whose user IDs do not conform to the permitted character set, and
 a number of rooms whose history includes events with a `sender` which
 does not conform. In order to handle these rooms successfully, clients
 and servers MUST accept user IDs with localparts consisting of any legal
-unicode codepoint except for `:`, including zero characters. Localparts
+unicode codepoint except for `:`, including the empty string. Localparts
 MUST be valid UTF-8 sequences.
 
 Servers SHOULD NOT produce user IDs with localparts outside of the following
@@ -674,7 +674,7 @@ by clients.
 
 The localpart of a room ID (`opaque_id` above) may contain any valid
 unicode codepoints except `:`, but it is recommended to only include
-ASCII letters and digits when generating them.
+ASCII letters and digits (`A-Z`, `a-z`, `0-9`) when generating them.
 
 #### Room Aliases