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.