CLIENT-SPECIFICATION: rephrase requirement to implement option variants (#15772)

Rephrase requirement to implement option variants

The new phrasing does not imply that all options have a short and long
form specified in the table.

CLIENT-SPECIFICATION.md

@@ -30,7 +30,11 @@ This section describes the standardised command-line interface (CLI) for clients
 
 ### Arguments
 
-The following table documents what command-line options MUST be supported and which are optional. If an option is implemented both the short and long versions MUST be available:
+The following table documents what command-line options MUST be supported and which are optional.
+
+When adding support for an option, clients MUST implement all variants of that option listed in the table.
+For example, clients should implement _both_ `-v` and `--version`.
+When a client implements updating the offline cache, they should support _both_ `-u` and `--update`.
 
 Option                 | Required?   | Meaning
 -----------------------|-------------|----------