ExternalProject: Avoid _Add() example in documentation

It messes with the self-extraction of the keyword list.
ExternalProject: Document multiple COMMAND lines

We support multiple commands per external project step by using
the COMMAND keyword.  Document this behavior and show an example.
While at it, document that shell operators and current working
directory behavior is not defined.
Merge topic 'doc-ExternalProject-COMMAND-keyword' into next

455b773 ExternalProject: Avoid _Add() example in documentation
Merge topic 'doc-ExternalProject-COMMAND-keyword' into next

93c1165 ExternalProject: Document multiple COMMAND lines