Help: Format execute_process command documentation

Use a code-block for the signature.  Format the options as a definition
list.
Help: Clarify execute_process COMMAND argument behavior

State in the execute_process how COMMAND arguments are passed to
the child.  Add a note at the end about how the command differs
from add_custom_*().

Suggested-by: Alan W. Irwin
Merge topic 'doc-execute_process' into next

3bb6266 Help: Clarify execute_process COMMAND argument behavior
ea6b656 Help: Format execute_process command documentation