To search through the man pages for some keywords, use the -k option.
man -k [keyword]
This shows a result of the commands, and routines that match the keyword with a one-line description of what they are about.
man -k passwd shows all the possible entries for passwd in the manual pages.
The apropos
The man -k [keyword] command is similar to a help utility called apropos which is available both on Unix and Linux. See it as a shortcut.