Make @ccall documentation appear in manual.

[fingolfin]

@ninjaaron added @ccall in PR #32748, including documentation for it in the form of docstrings.

However, searching for @ccall on https://docs.julialang.org/en/v1.6-dev/ does not produce any useful hits. I also tried finding it in there by browsing, but failed to find it. Perhaps something needs to be tweaked to actually make @ccall appear in the manual?

[ninjaaron]

I've been thinking about this also. There was some talk that @ccall should be considered the "default" approach for new code (or maybe that was just my delusion; can't remember), which would involve some pretty thoroughgoing rewrites, which is why I didn't make a PR about. However, maybe I should put some minimal documentation in the manual so there is something there and to get the discussion going about how much the existing documentation using ccall should be rewritten, if indeed it should be at all. I took some vacation time next week (more time for JuliaCon) and I think I can work on it then at some point.

[rfourquet]

As version 1.5 will be released very soon, before we have a chance to update thoroughly the manual, I think it would be good to have at least the docstring of the macro included somewhere in the manual for 1.5. I will tentatively add the 1.5 milestone to this issue.

[ninjaaron]

There's actually an error in the docstring as well! I noticed it yesterday as I was finishing up my lighting talk on the macro! The example with sprintf doesn't have an initial char * argument, which is the buffer where the formatted string is supposed to go! This could be fixed by simply changing it to printf.

[JeffBezanson]

This should be listed in doc/src/base/c.md.