Documented the usage of environment variables.

In particular the `OWNCLOUD_CHUNK_SIZE` as requested per #2449.
I looked through the code and it seems the env vars override the conf file, but I'm not 100 % sure.
The `OWNCLOUD_CRITICAL_FREE_SPACE_BYTES` and `OWNCLOUD_FREE_SPACE_BYTES` are now mentioned in two places.

doc/advancedusage.rst

@@ -14,6 +14,11 @@ Configuration File
 .. index:: config file
 .. include:: conffile.rst
 
+Environment Variables
+------------------
+.. index:: env vars
+.. include:: envvars.rst
+
 ownCloud Command Line Client
 ----------------------------
 .. index:: owncloudcmd

doc/envvars.rst

@@ -0,0 +1,11 @@
+The behavior of the client can also be controlled using environment variables. The value of the environment variables overrides the values in the configuration file.
+
+The environment variables are:
+
+- `OWNCLOUD_CHUNK_SIZE` (default: 5242880; 5 MiB) – Specifies the chunk size of uploaded files in bytes. Increasing this value may help with synchronization problems in certain configurations.  
+- `OWNCLOUD_TIMEOUT` (default: 300 s) – The timeout for network connections in seconds.
+- `OWNCLOUD_CRITICAL_FREE_SPACE_BYTES` (default: 50\*1000\*1000 bytes) - The minimum disk space needed for operation. A fatal error is raised if less free space is available. 
+- `OWNCLOUD_FREE_SPACE_BYTES` (default: 250\*1000\*1000 bytes) - Downloads that would reduce the free space below this value are skipped. More information available under the "Low Disk Space" section. 
+- `OWNCLOUD_MAX_PARALLEL` (default: 6) - Maximum number of parallel jobs. 
+- `OWNCLOUD_BLACKLIST_TIME_MIN` (default: 25 s) - Minimum timeout for blacklisted files.
+- `OWNCLOUD_BLACKLIST_TIME_MAX` (default: 24\*60\*60 s; one day) - Maximum timeout for blacklisted files.