More small edits for doc/lispref

More small edits for doc/lispref

* control.texi: Where possible, use example rather than smallexample.
(Sequencing, Conditionals, Signaling Errors, Handling Errors):
Tweak page breaks.

* customize.texi: Where possible, use example rather than smallexample.
(Common Keywords, Variable Definitions, Applying Customizations)
(Custom Themes): Tweak page breaks.

* eval.texi, functions.texi, loading.texi, macros.texi:
Where possible, use example rather than smallexample.

* sequences.texi (Arrays): Tweak page breaks.

* symbols.texi: Where possible, use example rather than smallexample.
(Symbol Components): Fix typo.
(Other Plists): Tweak page break.