git: document GIT_NO_REPLACE_OBJECTS environment variable

This variable is used as the primary way to disable the object
replacement mechanism, with the "--no-replace-objects" command line
option as an end-user visible way to set it, but has not been
documented.

The original reason why it was left undocumented might be because it
was meant as an internal implementation detail, but the thing is,
that our tests use the environment variable directly without the
command line option, and there certainly are folks who learned its
use from there, making it impossible to deprecate or change its
behaviour by now.

Add documentation and note that for this variable, unlike many
boolean-looking environment variables, only the presence matters,
not what value it is set to.

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

Documentation/git.txt

@@ -174,8 +174,10 @@ If you just want to run git as if it was started in `<path>` then use
 	directory.
 
 --no-replace-objects::
-	Do not use replacement refs to replace Git objects. See
-	linkgit:git-replace[1] for more information.
+	Do not use replacement refs to replace Git objects.
+	This is equivalent to exporting the `GIT_NO_REPLACE_OBJECTS`
+	environment variable with any value.
+	See linkgit:git-replace[1] for more information.
 
 --no-lazy-fetch::
 	Do not fetch missing objects from the promisor remote on
@@ -873,6 +875,10 @@ for full details.
 	header and packfile URIs. Set this Boolean environment variable to false to prevent this
 	redaction.
 
+`GIT_NO_REPLACE_OBJECTS`::
+	Setting and exporting this environment variable tells Git to
+	ignore replacement refs and do not replace Git objects.
+
 `GIT_LITERAL_PATHSPECS`::
 	Setting this Boolean environment variable to true will cause Git to treat all
 	pathspecs literally, rather than as glob patterns. For example,