Fix missing "if present" for "else"

Also other minor wording clarifications for if/then/else such as
standardizing the "regardless of the validation outcome against
the subschema" language.  That language is important as it
clarifies that subschema validation can fail (causing annotations
to be dropped) without causing the containing schema to fail.

Author: handrews

jsonschema-validation.xml

@@ -759,12 +759,13 @@
                     <t>
                         Instances that fail to validate against this
                         keyword's subschema MUST also be valid against
-                        the subschema value of the "else" keyword.
+                        the subschema value of the "else" keyword, if
+                        present.
                     </t>
                     <t>
                         Validation of the instance against this keyword
                         on its own always succeeds, regardless of the
-                        validation outcome of against its subschema.
+                        validation outcome of the instance against its subschema.
                     </t>
                 </section>
                 <section title="then">
@@ -780,8 +781,9 @@
                     <t>
                         When "if" is absent, or the instance fails to
                         validate against its subschema, validation against
-                        this keyword always succeeds.  Implementations
-                        SHOULD avoid attempting to validate against
+                        this keyword always succeeds, regardless of the
+                        validation outcome of the instance against its subschema.
+                        Implementations SHOULD avoid attempting to validate against
                         the subschema in these cases.
                     </t>
                 </section>
@@ -799,8 +801,9 @@
                     <t>
                         When "if" is absent, or the instance successfully
                         validates against its subschema, validation against
-                        this keyword always succeeds.  Implementations
-                        SHOULD avoid attempting to validate against
+                        this keyword always succeeds, regardless of the
+                        validation outcome of the instance against its subschema.
+                        Implementations SHOULD avoid attempting to validate against
                         the subschema in these cases.
                     </t>
                 </section>