docs: make sphinx ignore the environment cache to avoid missing synopsis

Pass the `-E` option to, quoting it's man-page, "don't use a saved
environment (the structure caching all cross-references, but rebuild
it completely."

As with reusing the environment one gets some empty results for
synopsis stuff depending on build order, for example the synopsis in
the command-syntax appendix HTML output is empty while the same
synopsis used for the dedicated HTML page is complete.

By making the build-log more verbose I caught the attention of some
emitted 'env-purge-doc' events from sphinx; while this itself might be
harmless (I didn't followed the rat tail to its end), it made me a bit
suspicious about caching and wrong/missing invalidation.

With ignoring the environment this is fixed, a diffoscope comparison
shows that not only the command-syntax page, but many others have the
various synposis content added again. There are solely added lines, no
removed nor changed, so it seems fine to enabled that option without
an in-depth sphinx review.

Note, I first suspected the use of a separate "doctree pickles" cache
directory (`-d` option) and is used for all output types besides the
man-pages one, which uses the default .doctree directory.
But changing the man-page target to also use the custom doctree cache
had no effect on the build-result whatsoever (compared with
diffoscope).

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/Makefile

@@ -89,7 +89,7 @@ API_VIEWER_FILES :=							\
 	/usr/share/javascript/proxmox-widget-toolkit-dev/APIViewer.js	\
 
 # Sphinx documentation setup
-SPHINXOPTS    =
+SPHINXOPTS    = -E
 SPHINXBUILD   = sphinx-build
 BUILDDIR      = output