Vim has a “built-in” functionality of displaying man pages. To activate this, add this to your .vimrc
runtime ftplugin/man.vim
This will load a standard plugin shipped with vim. You are now able to view the man page of a command with the vim command :Man. For example, if you want to know what the c function sbrk does, type
:Man 2 sbrk
If the function is already in the buffer, put the cursor on the function and hit K (as in Shift-k) to view the manual page of it. This only works on functions without hyphens.