Man pages for new `cargo report *` commands

[weihanglo]

Problem

The new unstable commands

• cargo report sessions
• cargo report rebuilds
• cargo report timings

don't have their own command documentation like other commands.

We'll need to follow how we document cargo report future-incompatibilties in #16430 in order to document them.

Proposed Solution

Document these commands by the same approach of #16430.

Notes

Blocked on #16430.