Clarify exists(pattern) function limitations

[gkorland]

Context

Migrated from FalkorDB/FalkorDB#1315.

Issue

The functions documentation describes the exists() predicate function as:

exists(pattern) — Returns true when at least one match for pattern exists

This implies that any graph pattern is supported. However, in practice, attempting to use exists() with a relationship pattern returns an error:

GRAPH.QUERY g "MATCH (n) WHERE exists((n)-[]-()) RETURN *"
(error) Unable to resolve filtered alias '(n)-[]-()'

Verified on FalkorDB v4.16.7 — the error still occurs.

Suggested Fix

Either:

1. Update the docs to clarify what pattern types exists() actually supports (e.g., only property existence checks like exists(n.prop), or only specific pattern forms), or
2. Note this as a known limitation and link to the related bug in the FalkorDB repo.

The WHERE clause docs show that pattern predicates work in WHERE directly (e.g., WHERE (p)-[:WON]->(s)), so users may be confused about why the same pattern doesn't work inside exists().