- Add missing links

- Fix extra ] spotted in online doc by emigas.
- Remove another ] introduced in Hearder syntax in r1588244
- s/variable/varname/ to match the text below it.
- Turn the syntax of both Header and RequestHeader to be:
   header [value [replacement [early|env=[!]variable|expr=expression]]] 
in order to match the code.
This removes the ambiguity of expr=<something> which can now be used in 2 different places in the syntax.
- Also remove the space in 'expr= value' in compatibility note, to help distinguish between the two

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1593664 13f79535-47bb-0310-9956-ffa450edef68