For major modes that apply to languages which have documentation in Info, you can use
C-h S (
info-lookup-symbol) to view the Info documentation for a symbol used in the program. You specify the symbol with the minibuffer; the default is the symbol appearing in the buffer at point. For example, in C mode this looks for the symbol in the C Library Manual. The command only works if the appropriate manual’s Info files are installed.
Emacs determines where to look for documentation for the symbol—which Info files to look in, and which indices to search—based on the major mode. You can also use
M-x info-lookup-file to look for documentation for a file name.
If you use
C-h S in a major mode that does not support it, it asks you to specify the symbol help mode. You should enter a command such as
c-mode that would select a major mode which
C-h S does support.