repl: standardize Control key indications

Throughout our messages and docs, we refer to the Control key in a surprisingly varied number of ways:

• Control
• Ctrl
• Cntl
• varied capitalization on the above (e.g., ctrl vs. Ctrl)

Then, in key combinations:

• One of the items from the previous list followed by -
• ... or followed by +
• ... surrounded or not by < and >
• ... and inside backticks or not
• ... or just ^

This is the start of standardization on the formulation recommended by the Microsoft Style Guide (e.g., Ctrl+C).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines