Add local cache expiry documentation and fix misleading error message when specifying a percentage cache quota

This patch aims to partially solve #700 by adding a section of to the documentation showing how we set local cache quotas in our user configuration.

It also fixes a misleading error message which occurs if your disk space quota (in %) exceeds the available disk space on the file system which contains the cache.

Output examples below:

Before this patch:

 $ cat ~/.config/buildstream.conf 
cache:
  quota: 90%

 $ df -h /
Filesystem                 Size  Used Avail Use% Mounted on
/dev/mapper/cryptAntergos  432G   47G  363G  12% /

 $ bst build hello.bst 

Error instantiating artifact cache: Your system does not have enough available space to support the cache quota specified.
You currently have:
- 8G of cache in use at /home/jennis/.cache/buildstream/artifacts
- 363G of available system storage

Clearly the above error is not true, I have 363G left. I think the intended behaviour is to say "Start cleaning once my disk space hits that percentage".

After this patch:

 $ cat ~/.config/buildstream.conf 
cache:
  quota: 90%

 $ df -h /
Filesystem                 Size  Used Avail Use% Mounted on
/dev/mapper/cryptAntergos  432G   48G  362G  12% /

 $ bst build hello.bst 

Error instantiating artifact cache: Your system does not have enough available space to support the cache quota specified.

You have specified a quota of 90% of total disk space.
- The filesystem containing /home/jennis/.cache/buildstream/artifacts only has: 83.8% of total disk space available.