Lack of clarity in description of directory variables

[Reuben Thomas]

> From the 1.9 manual:

`datarootdir'
     The root of the directory tree for read-only
     architecture-independent data files.  This should normally be
     `/usr/local/share', but write it as `$(prefix)/share'.
     [...]

`datadir'
     [...]
     This should normally be `/usr/local/share', but write it as
     `$(datarootdir)'.  (If you are using Autoconf, write it as
     address@hidden@.)

and similar for all the other variables under "Directory Variables". What 
puzzles me is the phrases "but write it as...": why in each case do you have 
to write the variable by using some other variable?

A possible clue is given under the entry for `exec_prefix', where it says 
instead:

     The default value of `exec_prefix' should be `$(prefix)'.

In other words, you're saying how to write *the default value* of the 
variable in each case, not *the variable itself*. Hence, I would change a 
sentence such as

     This should normally be `/usr/local/share', but write it as
     `$(datarootdir)'.

to

     This defaults to `$(datarootdir)' (which is normally
     `/usr/local/share').

If this needs more work to achieve both correctness and clarity I'd be happy 
to help.