Document index and index-has-key constraint (#494)

* Document index, index-has-key constraint for #490.

* Fix typo and key-field notation

1. Fix uniqueness para from the each member to that each member to make
the sentence grammatical.
1. Make all key-field references consistent to be `<key-field/>`, not
just `<key-field/>` to imply this child element can have children and is
different from a top-level `index` or `index-has-key` constraint.

* PR feedback edits: use Metaschema terms

* Woops, add in-page anchors to constraint listings

website/content/specification/syntax/constraints.md

@@ -18,7 +18,7 @@ The following constraint types are allowed for `<define-flag>` definitions.
 
 - [`<allowed-values>`](#enumerated-values)
 - `<matches>`
-- `<index-has-key>`
+- [`<index-has-key>`](#index-has-key-constraints)
 - `<expect>`
 
 For each of these constraint types, use of the `@target` attribute is prohibited. This is because a flag constraint may only target the flag, since a flag has no child nodes.
@@ -29,7 +29,7 @@ The following constraint types are allowed for `<define-field>` definitions.
 
 - [`<allowed-values>`](#enumerated-values)
 - `<matches>`
-- `<index-has-key>`
+- [`<index-has-key>`](#index-has-key-constraints)
 - `<expect>`
 
 ## `<define-assembly>` constraints
@@ -38,9 +38,9 @@ The following constraint types are allowed for `<define-assembly>` definitions.
 
 - [`<allowed-values>`](#enumerated-values)
 - `<matches>`
-- `<index-has-key>`
+- [`<index-has-key>`](#index-has-key-constraints)
 - `<expect>`
-- `<index>`
+- [`<index>`](#index-constraints)
 - `<is-unique>`
 - `<has-cardinality>`
 
@@ -179,6 +179,30 @@ A constraint may have an OPTIONAL [`@level`](#level) attribute and/or an OPTIONA
 
 If defined, the `<message>` value MUST be a [Metaschema string value](/specification/datatypes#string). It MAY contain a Metapath expression templates that starts with `{`, contains a Metapath expression, and ends with `}`.  When evaluating a template Metapath expression, the context of the Metapath [evaluation focus](#constraint-processing) MUST be the failing value node.
 
+## `index` constraints
+
+The `index` constraint is a type of Metaschema constraint that defines an index of document instance nodes addressable by key.
+
+The `@name` flag of an `<index>` constraint specifies the identity of the index. The constraint MUST define the name.
+
+The `@target` flag of an `<index>` constraint defines the node(s) in a document instance to index. The index MUST define a [`@target`](#target) with a Metapath expression. The processor MUST index only The document instance node(s) resulting from its evaluation.
+
+The `<key-field>` assembly of an `<index>` constraint defines the flag or field value that is the key for each entry in the index. The `<key-field>` field MUST define a [`@target`](#target) flag with a Metapath expression evaluated relative to [the evaluation focus](#constraint-processing) of each entry of the matches of the constraint's `@target`.
+
+An `index` constraint MAY define more than one `<key-field>` assembly. The composite key for each entry in the index is the combination of values for the `@target` of every `<key-field/>`. The composite values of the key are the discriminator for the uniqueness of the index entry.
+
+An `index` constraint requires that each member entry be unique based upon this composite key.
+
+If the evaluation of the Metapath `@target` of the `<key-field/>` does not result in a value, its value for that key in the index is null.
+
+## `index-has-key` constraints
+
+The `index-has-key` constraint is a type of Metaschema constraint that cross-references values an existing `index` constraint` with a separate `@target` and `<key-field/>`.
+
+The `@name` flag of an `<index>` constraint MUST specify the name of a previously defined `index` constraint.
+
+The `index-has-key` constraint has the same flags and assemblies as a [`index`](#index-constraints) constraint.
+
 ## Enumerated values
 
 The `allowed-values` constraint is a type of Metaschema constraint that restricts field or flag value(s) based on an enumerated set of permitted values.