params: Improve vcl_path documentation

[dridi]

In the varnishd(1) manual I see this:

   vcl_path
       NB:  The  actual  default value for this parameter depends on the Varnish Cache
       build environment and options.

          • Default: ${sysconfdir}/varnish:${datadir}/varnish/vcl

       Directory (or colon separated list of  directories)  from  which  relative  VCL
       filenames  (vcl.load and include) are to be found.  Includes using +glob cannot
       be searched in vcl_path.

       The actual default value of vcl_path depends on the  build  configuration.   It
       offers  standardized  directories  for VCL installation for packages and opera‐
       tors.

       In the second component ${datadir} expands to a directory like  /usr/share  for
       packaged  VCL.   This is where a VMOD or VEXT may install companion VCL, avail‐
       able directly as a relative include.

       In the first component ${sysconfdir} expands to a directory like /etc for oper‐
       ations.  This is where system administrators may install VCL files.

       If a VCL file installed under ${datadir}/varnish/vcl must be locally patched, a
       file with the same name can be created under ${sysconfdir}/varnish to  override
       it without interfering with the package that installed the original one.