`std::fmt` documentation does not explain the colon

[kpreid]

Location

https://doc.rust-lang.org/std/fmt/

Summary

The documentation of format strings uses the colon “:” without ever explaining what its role is. To some it will be obvious, but others will benefit from a brief explanation of the general syntax principle: that everything to the left of the “:” is the identifier of the input data, everything to the right is formatting options, and the colon per se does not change anything at all but only introduces the options.

@rustbot label +A-fmt

[lolbinarycat]

triage: fixed in #132393, cant add unit tests for docs, closing as completed.