Updated doc to cover use of temp directory on remote (#4545)

docs about remote temp directory usage

Updated doc to cover use of temp directory on remote, in particular how to handle changing the directory used for systems such as QNAP with tiny default /tmp directories.

fixup by tw: do not mention specific brands, specific amounts

this is not QNAP specific, but may happen as well on other limited
systems. the amount of tmp files can also vary widely.

docs/usage_general.rst.inc

@@ -404,7 +404,16 @@ Temporary files (client):
     won't be locally cached.
 
 Temporary files (server):
-    None.
+    A non-trivial amount of data will be stored on the remote temp directory
+    for each client that connects to it. For some remotes, this can fill the
+    default temporary directory at /tmp. This can be remediated by ensuring the
+    $TMPDIR, $TEMP, or $TMP environment variable is properly set for the sshd
+    process.
+    For some OSes, this can be done just by setting the correct value in the
+    .bashrc (or equivalent login config file for other shells), however in
+    other cases it may be neccessary to first enable ``PermitUserEnvironment yes``
+    in your ``sshd_config`` file, then add ``environment="TMPDIR=/my/big/tmpdir"``
+    at the start of the public key to be used in the ``authorized_hosts`` file.
 
 Cache files (client only):
     Contains the chunks index and files index (plus a collection of single-