bin/37822: make's IMPSRC should be better documented

[cheusov%tut.by@localhost]

>Number:         37822
>Category:       bin
>Synopsis:       IMPSRC should be better documented
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          doc-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jan 20 15:40:00 +0000 2008
>Originator:     cheusov%tut.by@localhost
>Release:        NetBSD 4.0_STABLE
>Organization:
>Environment:
System: NetBSD chen.chizhovka.net 4.0_STABLE NetBSD 4.0_STABLE (GENERIC) #2: 
Tue Dec 25 17:42:38 EET 2007 
cheusov%chen.chizhovka.net@localhost:/srv/obj/sys/arch/i386/compile/GENERIC i386
Architecture: i386
Machine: i386
>Description:
May be I missed something but the following Makefile
doesn't work as it is documented. 

man make says

             .IMPSRC The name/path of the source from which the target
                       is to be transformed (the ``implied'' source);
                       also known as `<'.

.IMPSRC is expanded to the empty string.

Something important is missed in the documentation.

~> cat Makefile 
file.ext : file.src
        cp ${.IMPSRC} ${.TARGET}
~> touch file.src
~> make file.ext  
cp  file.ext
usage: cp [-R [-H | -L | -P]] [-f | -i] [-pv] src target
       cp [-R [-H | -L | -P]] [-f | -i] [-pv] src1 ... srcN directory
*** Error code 1

Stop.
make: stopped in /home/cheusov
~>

>Fix:

Unknown