auto merge of #20782 : iKevinY/rust/trpl-formatting, r=steveklabnik

Here's my PR for the changes discussed in #19823. I decided to leave `_these_` types of italics the way there were because it differentiates the use of italics for emphasis from `*key term*` italics. Otherwise, bolded terms have been changed to italics, and single and double quotes have been changed appropriately, depending on their context (my judgement may not be the best, though).

r? @steveklabnik (congratulations on #19897 being finalized and merged, by the way!)

diff --cc src/doc/trpl/macros.md
Simple merge