doc: process: Document process.mainModule

Instrumentation code might need to find out the entry point of the
process in a global context.
Documenting the existing process.mainModule to officially support this.

Fixes #7808

Signed-off-by: Fedor Indutny <fedor@indutny.com>
doc: typos in child_process

Signed-off-by: Fedor Indutny <fedor@indutny.com>
module: document important methods

Signed-off-by: Fedor Indutny <fedor@indutny.com>
Merge remote-tracking branch 'upstream/master'