handbook: Add git switch info for shallow clones

[zhmylove]

Some people do not read man git-clone(1) and complain about "a bug" in the ports part of our handbook. This PR clarifies the instruction on git switch with --depth 1

[concussious]

I wonder if you should reword example 4 and 5 instead, since the paragraph above the examples says (paraphrased) "were using --depth 1 but if you want it deeper just omit that from the following examples"?

[zhmylove]

@concussious nice catch! Thank you. Updated the PR

[sergio-carlavilla]

@bsdimp what do you think about these changes? Can I commit them? Thanks for the review.

[bsdimp]

OK. I'm sold. Sorry it's taken me a while to get back to this.

[bsdimp]

50735390bd lands the change.