add clarifying language around the use of global variables

[FajitaOfTreason]

updated variables guide page:

• fix a few broken links on variables page
• clarify the language around global variable use to hopefully help avoid common mistakes
• add links to get / set global sub-action pages
  updated set / get global sub-action pages:
• remove obsolete Argument option
• remove inaccurate camelCase warning
• add warning around using % symbols in variable name fields
  added a line break to If/Else page to increase prominence of the Does Not Exist variable name part of the parsing warning