usage: in doc strings. Correct side-effect function remark.

lispref/internals.texi

@@ -563,6 +563,8 @@ If the last line of the documentation string begins with the keyword
 @samp{usage:}, the rest of the line is treated as the argument list
 for documentation purposes.  This way, you can use different argument
 names in the documentation string from the ones used in the C code.
+@samp{usage:} is required if the function has an unlimited number of
+arguments.
 
 All the usual rules for documentation strings in Lisp code
 (@pxref{Documentation Tips}) apply to C code documentation strings
@@ -746,10 +748,10 @@ number of arguments.  They work by calling @code{Ffuncall}.
 @file{lisp.h} contains the definitions for some important macros and
 functions.
 
-  If you define a function which is side-effect free, update the code in
-@file{byte-opt.el} which binds @code{side-effect-free-fns} and
-@code{side-effect-and-error-free-fns} to include it.  This will help the
-optimizer.
+  If you define a function which is side-effect free, update the code
+in @file{byte-opt.el} which binds @code{side-effect-free-fns} and
+@code{side-effect-and-error-free-fns} so that the compiler optimizer
+knows about it.
 
 @node Object Internals
 @appendixsec Object Internals