update dvcignore docs

[pared]

With merging of treeverse/dvc#2161
dvcignored is supported in every walk in the code.
Reflect that in docs.

[shcheklein]

@pared please, give a little bit more details on how it should be updated. It's hard to put walk into some actionable/writable context for us :)

[efiop]

@pared ?

[pared]

@shcheklein , @efiop , sorry must have lost this one in my notifications. It will probably be the best if I do this change. Probably summary of what needs to be written down is not so far away from actual change.