add description of the pull option to outs in .dvc files

[pbailey-hf]

This is the documentation update that goes with iterative/dvc#10451

[shcheklein]

optionally, it would be nice to update the pull, checkout, fetch pages also in some relevant places. e.g. this paragrpah:

Without arguments, it downloads all files and directories referenced in the current workspace ...

we can mention there something like (except for outputs that have push: false set, see ...)

also if there are some paragraphs about explicit targets - I would consider adding a small not there