git-fetch: add example for updating another branch

[waldyrious]

Checklist

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Reference issue: #

[sbrl]

.....but wouldn't this now say that the branch can't be changed, since it isn't in {{braces}}, @Managor?

[Managor]

No. The point is to match the description. If the description is explixit, then the example command is explicit as well.

[waldyrious]

For the record, I do agree with @sbrl that it feels awkward to not mark the "main" as placeholders just because the description refers to that explicitly.

I believe we are yet to define a single standard for how to specify command arguments —i.e. use concrete examples or more generic placeholder names— and whether to mark that difference using the placeholder syntax. For example, it feels almost backward that in the sed page, we have the following example:

- [p]rint only the first line to `stdout`:

`{{command}} | sed {{[-n|--quiet]}} '1p'`

It's almost as we're saying that the part that is core to that example, in terms of determining its behavior (i.e. the -n option) is replaceable, whereas the actual variable bit (1p) is hardcoded.

But hey, that's a discussion for another time and place. I just wanted to make my thoughts explicit since the point was raised here.