Documentation: clarify "git clean -e <pattern>"

Documentation: clarify "git clean -e <pattern>"

The current explanation of -e can be misread as allowing the user to say

    I know 'git clean -XYZ' (substitute -XYZ with any option and/or
    parameter) will remove paths A, B, and C, and I want them all removed
    except for paths matching this pattern by adding '-e C' to the same
    command line, i.e. 'git clean -e C -XYZ'.

But that is not what this option does. It augments the set of ignore rules
from the command line, just like the same "-e <pattern>" argument does
with the "ls-files" command (the user could probably pass "-e \!C" to tell
the command to clean everything the command would normally remove, except
for C). Also error out when both -x and -e are given with an explanation of
what -e means---it is a symptom of misunderstanding what -e does.

It also fixes small style nit in the parameter to add_exclude() call. The
current code only works because EXC_CMDL happens to be defined as 0.

Signed-off-by: Junio C Hamano <gitster@pobox.com>