Call useWithStale instead of useWithStaleFast when calling ParseCabalFields

[VeryMilkyJoe]

Also adds some documentation about why we use each of these versions of use.
Fixes #4293.

[michaelpj]

This doesn't seem right to me. I thought completions were one of the typical examples where we definitely want fast response speed at the expense of stale output?

[fendor]

In general true, but readFields is incredibly fast as it is basically a single pass over the file contents.
In blow testing, this felt like the better default beaviour..