Systematize documentation of environment variables

Cache locations were documented inconsistently in three places.

The backup docs mentioned PATH being used to find fusermount, which is
never run by restic backup. It now mentions ssh and rclone, which are
used by backends.

The notion of a "system-wide" environment variable makes no sense.
TMPDIR is now mentioned because it allows for optimization and may
have security implications.

doc/040_backup.rst

@@ -416,6 +416,8 @@ environment variables. The following lists these environment variables:
     RESTIC_CACHE_DIR                    Location of the cache directory
     RESTIC_PROGRESS_FPS                 Frames per second by which the progress bar is updated
 
+    TMPDIR                              Location for temporary files
+
     AWS_ACCESS_KEY_ID                   Amazon S3 access key ID
     AWS_SECRET_ACCESS_KEY               Amazon S3 secret access key
     AWS_DEFAULT_REGION                  Amazon S3 default region
@@ -453,12 +455,12 @@ environment variables. The following lists these environment variables:
 
     RCLONE_BWLIMIT                      rclone bandwidth limit
 
-In addition to restic-specific environment variables, the following system-wide environment variables
-are taken into account for various operations:
+See :ref:`caching` for the rules concerning cache locations when
+``RESTIC_CACHE_DIR`` is not set.
 
- * ``$XDG_CACHE_HOME/restic``, ``$HOME/.cache/restic``: :ref:`caching`.
- * ``$TMPDIR``: :ref:`temporary_files`.
- * ``$PATH/fusermount``: Binary for ``restic mount``.
+The external programs that restic may execute include ``rclone`` (for rclone
+backends) and ``ssh`` (for the SFTP backend). These may respond to further
+environment variables and configuration files; see their respective manuals.
 
 
 Exit status codes

doc/cache.rst

@@ -3,23 +3,13 @@ Local Cache
 ***********
 
 In order to speed up certain operations, restic manages a local cache of data.
-This document describes the data structures for the local cache with version 1.
+The location of the cache directory depends on the operating system and the
+environment; see :ref:`caching`.
 
-Versions
-========
-
-The cache directory is selected according to the `XDG base dir specification
-<https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html>`__.
 Each repository has its own cache sub-directory, consisting of the repository ID
 which is chosen at ``init``. All cache directories for different repos are
 independent of each other.
 
-The cache dir for a repo contains a file named ``version``, which contains a
-single ASCII integer line that stands for the current version of the cache. If
-a lower version number is found the cache is recreated with the current
-version. If a higher version number is found the cache is ignored and left as
-is.
-
 Snapshots, Data and Indexes
 ===========================
 

doc/manual_rest.rst

@@ -398,10 +398,14 @@ This allows faster operations, since meta data does not need to be loaded from
 a remote repository. The cache is automatically created, usually in an
 OS-specific cache folder:
 
- * Linux/other: ``~/.cache/restic`` (or ``$XDG_CACHE_HOME/restic``)
+ * Linux/other: ``$XDG_CACHE_HOME/restic``, or ``~/.cache/restic`` if
+   ``XDG_CACHE_HOME`` is not set
  * macOS: ``~/Library/Caches/restic``
  * Windows: ``%LOCALAPPDATA%/restic``
 
+If the relevant environment variables are not set, restic exits with an error
+message.
+
 The command line parameter ``--cache-dir`` or the environment variable
 ``$RESTIC_CACHE_DIR`` can be used to override the default cache location.  The
 parameter ``--no-cache`` disables the cache entirely. In this case, all data