There is no formal definition of a manpage anywhere, not even in the POSIX standard. The man(1) manpage in your example is pretty typical: you write out the various ways a program can be used (often just one) with [] denoting optional, bold (or typewriter font with the mdoc macros) denoting literal command line input and italics denoting variables.
The manpages man(7) and mdoc(7) will explain the most important conventions. man(7) is for old-style Unix manpages and is still popular on Linux (see man-pages(7)); mdoc(7) comes from 4.4BSD and is popular in its derivatives. The latter maintains a stricter separation of content and presentation and can produce (IMHO) prettier PDF/HTML output