docs(rules): document the bladed dash approximation

Bladed Dash's description now matches the SRD (standard action, move up
to 30 ft in a straight line without provoking attacks of opportunity,
single melee attack at highest BAB against any creature adjacent along
the path) and the README's spell-limits section records the engine
approximation: a +30 ft speed buff for 1 round, with the free melee
attack, the no-AoO movement, the straight-line constraint and the
circumstance attack bonus left unmodeled.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

data/spells/bladed_dash.json

@@ -15,5 +15,5 @@
       "duration_rounds": 1
     }
   ],
-  "description": "You move up your speed as a move action and make one melee attack at the end. Simplified as a +30 ft speed buff for 1 round; the extra attack is not modeled."
+  "description": "Upon casting, you immediately move up to 30 feet in a straight line in any direction; this movement does not provoke attacks of opportunity, and you may make a single melee attack at your highest base attack bonus against any creature you are adjacent to at any point along this 30 feet (SRD bladed dash). Simplified as a +30 ft speed buff for 1 round; the free melee attack, the no-AoO movement, the straight-line constraint and the circumstance attack bonus are not modeled."
 }