C-h a is bound to command apropos-command. C-h k C-h a tells us:
C-h a runs the command apropos-command (found in global-map), which is
an autoloaded interactive Lisp function in ‘apropos.el’.
It is bound to C-h a, <f1> a, <help> a, <menu-bar> <help-menu> <search-documentation> <find-commands-by-name>.
(apropos-command PATTERN &optional DO-ALL VAR-PREDICATE)
Show commands (interactively callable functions) that match PATTERN.
PATTERN can be a word, a list of words (separated by spaces),
or a regexp (using some regexp special characters). If it is a word,
search for matches for that word as a substring. If it is a list of words,
search for matches for any two (or more) of those words.
With C-u prefix, or if apropos-do-all is non-nil, also show
noninteractive functions.
If VAR-PREDICATE is non-nil, show only variables, and only those that
satisfy the predicate VAR-PREDICATE.
When called from a Lisp program, a string PATTERN is used as a regexp,
while a list of strings is used as a word list.
In addition, there are other apropos commands that help. For example, C-h d is bound to apropos-doc:
apropos-documentation is an autoloaded interactive byte-compiled Lisp
function in ‘apropos.el’.
It is bound to C-h d, <f1> d, <help> d, <menu-bar> <help-menu> <search-documentation> <search-documentation-strings>.
(apropos-documentation PATTERN &optional DO-ALL)
Show symbols whose documentation contains matches for PATTERN.
PATTERN can be a word, a list of words (separated by spaces),
or a regexp (using some regexp special characters). If it is a word,
search for matches for that word as a substring. If it is a list of words,
search for matches for any two (or more) of those words.
Note that by default this command only searches in the file specified by
internal-doc-file-name; i.e., the etc/DOC file. With C-u prefix,
or if apropos-do-all is non-nil, it searches all currently defined
documentation strings.
Returns list of symbols and documentation found.
