bug#6486: documentation of `byte-code-function-p' should mention `symbol-function' and xref manual

[MON KEY]

The documentation of function `byte-code-function-p' should mention
usage requires `symbol-function', and/or should refer to a relevant
portion of the manual.

,---- (documentation 'byte-code-function-p)
|
| Return t if OBJECT is a byte-compiled function object.
|
| (fn OBJECT)
|
`----

The above docs do not adequately indicate that the OBJECT arg is as
per the return value of `symbol-function'. e.g.:

 (byte-code-function-p 'disassemble)
 ;=> nil

 (byte-code-function-p (symbol-function 'disassemble))
 ;=> t

How is the user supposed to know that OBJECT will only return t if it
is the unreadable vector returned by `symbol-function'?

Please add documentation of such, along with info node xref such as:

 See info node `(elisp) Byte-Code Objects'

Also, note that the nature of the data-structure/readability of a
byte-code'd function can not be deduced by the user by simply reading
the manual section:

 (info "(elisp)What Is a Function")

--
/s_P\