style-guide: Avoid using "should" or "may" for required parts of the default style

The style guide inconsistently used language like "there should be a
space" or "it should be on its own line", or "may be written on a single
line", for things that are required components of the default Rust
style. "should" and especially "may" come across as optional. While the
style guide overall now has a statement at the top that the default
style itself is a *recommendation*, the *definition* of the default
style should not be ambiguous about what's part of the default style.

Rewrite language in the style guide to only use "should" and "may" and
similar for truly optional components of the style (e.g. things a tool
cannot or should not enforce in its default configuration).

In their place, either use "must", or rewrite in imperative style ("put
a space", "start it on the same line"). The latter also substantially
reduces the use of passive voice.

This is a purely editorial change, and does not affect the semantic
definition of the Rust style.

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