Improve pure and side-effect-free docs

For discussion, see thread starting at:
https://lists.gnu.org/archive/html/emacs-devel/2019-04/msg00316.html
* doc/lispref/customize.texi (Composite Types): Do not overspecify
:match-alternatives predicates.
* doc/lispref/eval.texi (Intro Eval): Anchor definition of "side
effect" for cross-referencing...
* doc/lispref/functions.texi (What Is a Function): ...from here.
Define what a pure function is.
* doc/lispref/internals.texi (Writing Emacs Primitives): Describe
currently preferred approach to marking primitives as pure and
side-effect-free.
* doc/lispref/symbols.texi (Standard Properties): Expand description
of pure and side-effect-free properties.

1 files changed, 4 insertions, 4 deletions

diff --git a/doc/lispref/customize.texi b/doc/lispref/customize.texi
index f71dedfd8b0..02eefe0f585 100644
--- a/doc/lispref/customize.texi
+++ b/doc/lispref/customize.texi
@@ -950,10 +950,10 @@ possibilities:
 
 @itemize @bullet
 @item
-A predicate---that is, a function of one argument that has no side
-effects, and returns either @code{nil} or non-@code{nil} according to
-the argument.  Using a predicate in the list says that objects for which
-the predicate returns non-@code{nil} are acceptable.
+A predicate---that is, a function of one argument that returns either
+@code{nil} or non-@code{nil} according to the argument.  Using a
+predicate in the list says that objects for which the predicate
+returns non-@code{nil} are acceptable.
 
 @item
 A quoted constant---that is, @code{'@var{object}}.  This sort of element