var doc: default editor and pager are configurable at build time

Some distributors customize the fallback pager and editor used by git
commands when the user has not indicated a preference via the
core.editor/core.pager configuration or GIT_EDITOR, GIT_PAGER, VISUAL,
EDITOR, and PAGER environment variables, and git's build system
provides DEFAULT_PAGER and DEFAULT_EDITOR makefile settings to help
them with that (see v1.6.6-rc0~24, 2009-11-20).

Unfortunately those compile-time settings do not affect the
documentation, so the uninitiated user who tries to understand git by
reading the git-var(1) manpage can easily be confused when git falls
back to 'nano' and 'more' instead of 'vi' and 'less'.  Even if the
distributor patches the distributed docs to reflect the new default,
the user may read the official documentation from the git-htmldocs
repository online and be confused in the same way.

Add a few words stating that the defaults are customizable at
compile time to make the behavior crystal clear.

Reported-by: Rodrigo Silva (MestreLion) <linux@rodrigosilva.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-var.txt

@@ -43,13 +43,15 @@ GIT_EDITOR::
     `$SOME_ENVIRONMENT_VARIABLE`, `"C:\Program Files\Vim\gvim.exe"
     --nofork`.  The order of preference is the `$GIT_EDITOR`
     environment variable, then `core.editor` configuration, then
-    `$VISUAL`, then `$EDITOR`, and then finally 'vi'.
+    `$VISUAL`, then `$EDITOR`, and then the default chosen at compile
+    time, which is usually 'vi'.
 
 GIT_PAGER::
     Text viewer for use by git commands (e.g., 'less').  The value
     is meant to be interpreted by the shell.  The order of preference
     is the `$GIT_PAGER` environment variable, then `core.pager`
-    configuration, then `$PAGER`, and then finally 'less'.
+    configuration, then `$PAGER`, and then the default chosen at
+    compile time (usually 'less').
 
 Diagnostics
 -----------