docs: Document that TMP/TEMP/USERPROFILE are consulted for temporary directory on windowswip/tmpdir

It confused me that the TMPDIR variable worked on windows but not on
linux. Our implementation relies on the directory implementation of
`getTemporaryDirectory` which consults these variables so I have
reflected that in our documentation.

1 files changed, 11 insertions, 3 deletions

diff --git a/docs/users_guide/separate_compilation.rst b/docs/users_guide/separate_compilation.rst
index abcc2f3e31..123ea017fc 100644
--- a/docs/users_guide/separate_compilation.rst
+++ b/docs/users_guide/separate_compilation.rst
@@ -542,9 +542,17 @@ Redirecting temporary files
     .. index::
         single: TMPDIR environment variable
 
-    Alternatively, use your :envvar:`TMPDIR` environment variable. Set it to the
-    name of the directory where temporary files should be put. GCC and other
-    programs will honour the :envvar:`TMPDIR` variable as well.
+    Alternatively, on unix, use your :envvar:`TMPDIR` environment variable. Set it to the
+    name of the directory where temporary files should be put.
+
+    On Windows the environment variables :envvar:`TMP`, :envar:`TEMP`,
+    :envvar:`USERPROFILE` are consulted in that order in order to determine the
+    directory for temporary files.
+
+    The cross-platform way to specify the temporary directory is to use the
+    command-line flag.
+
+
 
 .. _hi-options: