docs(tutorial): add pre-v2.3 GIT_SSH example

It goes along with the new one advertising the GIT_SSH_COMMAND
environment variable.

Related to #256

Author: Byron

Diff for: doc/source/tutorial.rst

@@ -331,11 +331,17 @@ You can easily access configuration information for a remote by accessing option
     :start-after: # [26-test_references_and_objects]
     :end-before: # ![26-test_references_and_objects]
 
-You can also specify per-call custom environments using a new context manager on the Git command, e.g. for using a specific SSH key.
+You can also specify per-call custom environments using a new context manager on the Git command, e.g. for using a specific SSH key. The following example works with `git` starting at *v2.3*.
 
     ssh_cmd = 'ssh -i id_deployment_key'
     with repo.git.custom_environment(GIT_SSH_COMMAND=ssh_cmd):
         repo.remotes.origin.fetch()
+
+This one sets a custom script to be executed in place of `ssh`, and can be used in `git` prior to *v2.3*.
+
+    ssh_executable = os.path.join(rw_dir, 'my_ssh_executable.sh')
+    with repo.git.custom_environment(GIT_SSH=ssh_executable):
+        repo.remotes.origin.fetch()
 
 Submodule Handling
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