bug#8119: 24.0.50; `mark-active' needs its doc string

[Glenn Morris]

retitle 8119 buggy 2011-02-14 trunk build on MS Windows
stop

> Emacs 24:
> "Not documented as a variable."

s/Emacs 24/current trunk/

Anyway, buggy build. 

Current trunk on GNU/Linux:
    
    mark-active is a variable defined in `C source code'.
    Its value is nil
    Local in buffer *scratch*; global value is nil
    
      Automatically becomes buffer-local when set in any fashion.
    
    Documentation:
    Non-nil means the mark and region are currently active in this buffer.