Document $inverseMatch, and close out the missing-stages list

[GuanzhouSong]

Addresses the remainder of documentdb/documentdb.github.io#130. Nine of the fourteen stages it listed were documented in #59 and #64; this covers the five that were left, though not in the way the issue expects — only one of them can currently be run.

The one that ships: $inverseMatch

Implemented, ungated, and specific to DocumentDB — there is no MongoDB equivalent, so a reader who meets it in a pipeline has nowhere else to look. That makes it the highest-value page of the original fourteen.

The stage inverts the direction of a match: $match holds one query and tests it against many documents, while $inverseMatch reads a query out of each document and tests it against a supplied input. It is the stage for collections of stored queries — saved searches, alert rules, subscription filters.

Documented from ParseInverseMatchSpec and the operator implementation: required path, mutually exclusive input / from, pipeline required alongside from and restricted to $match, $project and $limit, and defaultResult governing documents that carry no query. Two behaviours got explicit space because both are quietly surprising — a missing path is not an error but a silent drop unless defaultResult says otherwise, and input is parsed as an aggregation expression, so a bare string is a path reference rather than a literal document.

The four that do not: no pages, deliberately

Stage	Why not
$listLocalSessions	.mutateFunc = NULL
$listSessions	.mutateFunc = NULL
$searchMeta	.mutateFunc = NULL
$listSearchIndexes	Feature-flagged off, and its rewrite hook is unset in this build

A stage registered with a null handler is rejected by the pipeline builder itself (bson_aggregation_pipeline.c:9019):

if (definition->mutateFunc == NULL)
    ereport(ERROR, (errcode(ERRCODE_DOCUMENTDB_COMMANDNOTSUPPORTED),
        errmsg("Stage %s is not supported yet in native pipeline", definition->stage)));

$listSearchIndexes fails twice over. documentdb.enableExtendedIndexes defaults to false (feature_flag_configs.c:214), so it errors $listSearchIndexes stage is not enabled out of the box; and even with the flag on, rewrite_list_extended_indexes_query_hook is NULL in this build (api_hooks.c:47), so the handler's RewriteListExtendedIndexesQuery returns NULL and it errors $listSearchIndexes is not supported. It is an extension point for a downstream distribution, not a stage this engine can run.

Writing reference pages for these four would repeat the mistake #64 had to correct on $search: documenting MongoDB's behaviour for something DocumentDB does not implement, in a form that looks authoritative and fails on first use.

A note on the count

Issue #130 derived "40 public stages" from StageDefinitions[], and its own analysis warned that the table "gives a name and a .mutateFunc pointer, and says nothing about what the stage accepts". That caveat applies to the count as well: at least four entries in the table cannot be executed. The gap was smaller than it looked.

Verification

Every claim here comes from the engine source at v0.114-0, not from MongoDB's documentation. The three outbound links on the new page were checked against the live site and return 200.