Update naming.adoc

Signed-off-by: Kersten Richter <kersten@riscv.org>

src/naming.adoc

@@ -76,11 +76,13 @@ of the P extension.
 
 === Additional Standard Unprivileged Extension Names
 
-Standard unprivileged extensions can also be named using a single "Z" followed by
-an alphabetical name and an optional version number. For example,
+Standard unprivileged extensions can also be named using a single "Z" followed by an
+alphanumeric name, the name must end with an alphabetical, an optional
+version number, and the second letter from the bottom cannot be a numeric if the
+last letter is ``p''. For example,
 "Zifencei" names the instruction-fetch fence extension described in
 <<zifencei>>; "Zifencei2" and
-"Zifencei2p0" name version 2.0 of same.
+"Zifencei2p0" name version 2.0 are the same.
 
 The first letter following the "Z" conventionally indicates the most
 closely related alphabetical extension category, IMAFDQLCBKJTPVH. For the
@@ -99,10 +101,14 @@ separated from other multi-letter extensions by an underscore, e.g.,
 Standard extensions that extend the supervisor-level virtual-memory
 architecture are prefixed with the letters "Sv", followed by an alphabetical
 name and an optional version number, or by a numeric name with no version number.
-Other standard extensions that extend
-the supervisor-level architecture are prefixed with the letters "Ss",
-followed by an alphabetical name and an optional version number.  Such
-extensions are defined in Volume II.
+Other standard extensions that extend the supervisor-level architecture are 
+prefixed with the letters "Ss", followed by an alphabetical name and an 
+optional version number. Such extensions are defined in Volume II. In 
+general, they are named by using ``S'' as a prefix, followed by an 
+alphanumeric name. The name must end with an alphabetical, an optional 
+version number, and the second letter from the bottom cannot be a numeric 
+if the last letter is ``p''. Supervisor-level extensions must be separated 
+from other multi-letter extensions by an underscore.
 
 Standard supervisor-level extensions should be listed after standard
 unprivileged extensions. If multiple supervisor-level extensions are
@@ -132,10 +138,11 @@ listed, they should be ordered alphabetically.
 
 === Non-Standard Extension Names
 
-Non-standard extensions are named using a single "X" followed by an
-alphabetical name and an optional version number. For example,
-"Xhwacha" names the Hwacha vector-fetch ISA extension; "Xhwacha2"
-and "Xhwacha2p0" name version 2.0 of same.
+Non-standard extensions are named by using a single "X" followed by the alphanumeric 
+name. The name must end with an alphabetic character and an optional version number. The 
+second letter from the bottom cannot be a numeric if the last letter is 
+``p''. For example, "Xhwacha" names the Hwacha vector-fetch ISA 
+extension; "Xhwacha2" and "Xhwacha2p0" name version 2.0 of same.
 
 Non-standard extensions must be listed after all standard extensions, and,
 like other multi-letter extensions, must be separated from other multi-letter