bug#3135: 23.0.92; doc of custom-variable-documentation

[Drew Adams]

Doc string:
 
 Return documentation of VARIABLE for use in Custom buffer.
 Normally just return the docstring.  But if VARIABLE automatically
 becomes buffer local when set, append a message to that effect.
 
We need to make clear that VARIABLE need not be a user
variable. Especially because much of the time the doc for `custom-*'
treats a VARIABLE as a user variable.