Include pattern for validating resource ID. (#791)

aip/general/0122.md

@@ -115,7 +115,7 @@ ID for the publisher, and `les-miserables` is the resource ID for the book.
     which restricts to letters, numbers, and hyphen, with the first character
     a letter, the last a letter or a number, and a 63 character maximum.
     - Additionally, user-settable resource IDs **should** restrict letters to
-      lower-case.
+      lower-case (`^[a-z]([a-z0-9-]{0,61}[a-z0-9])?$`).
     - Characters outside of ASCII **should not** be permitted; however, if
       Unicode characters are necessary, APIs **must** follow guidance in
       [AIP-210][].

aip/general/0210.md

@@ -87,6 +87,8 @@ Unique identifiers **should** use a maximum length of 64 characters, though
 this limit may be expanded as necessary. 64 characters should be sufficient for
 most purposes as even UUIDs only require 36 characters.
 
+**Note:** See AIP-122 for recommendations about resource ID segments.
+
 ### Normalization
 
 **TL;DR:** Unicode values **should** be stored in [Normalization Form C][].