Minor selection clarifications

Minor selection clarifications

- Disallow unquoted strings as selection names.  This prevents
  "help all" getting parsed as a valid selection with name "help", as
  well as some unwanted behavior with typos in the first word of a
  selection.  The selections are much more readable with the name
  quoted, and all examples use the quotes, so there should be no harm
  done in making the syntax stricter in this case.
- Clarify the generic help text printed out by 'gmx help selections'.
  Now the help text references the manual, which contains a bit longer
  introduction to selections, instead of trying to duplicate that
  content.

Change-Id: I76a0b54b19e631c96c167cb55ed2dac5cff328a4