Clarified gitattributes documentation regarding custom hunk header.

The only part of the hunk header that we can change is the "TEXT"
portion.  Additionally, a few grammatical errors have been corrected.

Signed-off-by: Garry Dolley <gdolley@ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -270,27 +270,27 @@ See linkgit:git[1] for details.
 Defining a custom hunk-header
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Each group of changes (called "hunk") in the textual diff output
+Each group of changes (called a "hunk") in the textual diff output
 is prefixed with a line of the form:
 
 	@@ -k,l +n,m @@ TEXT
 
-The text is called 'hunk header', and by default a line that
-begins with an alphabet, an underscore or a dollar sign is used,
-which matches what GNU 'diff -p' output uses.  This default
-selection however is not suited for some contents, and you can
-use customized pattern to make a selection.
+This is called a 'hunk header'.  The "TEXT" portion is by default a line
+that begins with an alphabet, an underscore or a dollar sign; this
+matches what GNU 'diff -p' output uses.  This default selection however
+is not suited for some contents, and you can use a customized pattern
+to make a selection.
 
-First in .gitattributes, you would assign the `diff` attribute
+First, in .gitattributes, you would assign the `diff` attribute
 for paths.
 
 ------------------------
 *.tex	diff=tex
 ------------------------
 
-Then, you would define "diff.tex.funcname" configuration to
+Then, you would define a "diff.tex.funcname" configuration to
 specify a regular expression that matches a line that you would
-want to appear as the hunk header, like this:
+want to appear as the hunk header "TEXT", like this:
 
 ------------------------
 [diff "tex"]