docs: Small changes regarding compression

- Mention zstd as the best general choice when not using lz4
  (as often acknowledged by public benchmarks)
- Mention 'auto' more prominently as a good heuristic to improve
  speed while retaining good compression
- Link to compression options

(cherry picked from commit f656f6b1f24272c7eedf052984126c86e76eac73)

docs/quickstart.rst

@@ -311,36 +311,30 @@ Backup compression
 ------------------
 
 The default is lz4 (very fast, but low compression ratio), but other methods are
-supported for different situations.
+supported for different situations. Compression not only helps you save disk space,
+but will especially speed up remote backups since less data needs to be transferred.
 
-You can use zstd for a wide range from high speed (and relatively low
-compression) using N=1 to high compression (and lower speed) using N=22.
-
-zstd is a modern compression algorithm and might be preferable over zlib and
-lzma, except if you need compatibility to older borg versions (< 1.1.4) that
-did not yet offer zstd.::
+zstd is a modern compression algorithm which can be parametrized to anything between
+N=1 for highest speed (and relatively low compression) to N=22 for highest compression
+(and lower speed)::
 
     $ borg create --compression zstd,N /path/to/repo::arch ~
 
-Other options are:
-
-If you have a fast repo storage and you want minimum CPU usage, no compression::
+If you have a fast repo storage and you want minimum CPU usage you can disable
+compression::
 
     $ borg create --compression none /path/to/repo::arch ~
 
-If you have a less fast repo storage and you want a bit more compression (N=0..9,
-0 means no compression, 9 means high compression):
-
-::
+You can also use zlib and lzma instead of zstd, although zstd usually provides the
+the best compression for a given resource consumption. You may want to use these
+algorithms if you need compatibility to older borg versions (< 1.1.4) that
+did not yet offer zstd. Please see :ref:`borg_compression` for all options.
 
-    $ borg create --compression zlib,N /path/to/repo::arch ~
-
-If you have a very slow repo storage and you want high compression (N=0..9, 0 means
-low compression, 9 means high compression):
-
-::
+An interesting alternative is ``auto``, which first checks with lz4 whether a chunk is
+compressible (that check is very fast), and only if it is, compresses it with the
+specified algorithm::
 
-    $ borg create --compression lzma,N /path/to/repo::arch ~
+    $ borg create --compression auto,zstd,7 /path/to/repo::arch ~
 
 You'll need to experiment a bit to find the best compression for your use case.
 Keep an eye on CPU load and throughput.

src/borg/archiver.py

@@ -2672,7 +2672,8 @@ class Archiver:
             The heuristic tries with lz4 whether the data is compressible.
             For incompressible data, it will not use compression (uses "none").
             For compressible data, it uses the given C[,L] compression - with C[,L]
-            being any valid compression specifier.
+            being any valid compression specifier. This can be helpful for media files
+            which often cannot be compressed much more.
 
         obfuscate,SPEC,C[,L]
             Use compressed-size obfuscation to make fingerprinting attacks based on