docs: the explicit graph build warning in the vectors how-to is out of date after ArcadeDB #11130 #197

The vectors how-to (docs/modules/ROOT/pages/how-to/vectors.adoc, merged in #191) says that while build_graph_now() runs, searches on that index wait until it returns, and advises not to call it while other threads search the same index. That is no longer true from ArcadeDB 26.11.1.

ArcadeData/arcadedb#11129 reported that an explicit buildVectorGraphNow() on an LSM_VECTOR index made every search wait for the whole build when a graph was already in memory and nothing wrote during the build. ArcadeData/arcadedb#11130 fixed it (merged 2026-11-02, milestone 26.11.1). Searches now keep answering from the graph in memory plus the vectors written since, and switch to the new graph when it is ready, as during the automatic background rebuild. Only an index with no graph in memory yet (for example not searched since the database was opened) still waits.

Our measurement on the merged head, 25 s rebuild:

On the base commit, 1 (JDK 21) and 0 (JDK 25) searches completed, each waiting the whole 19 to 20 s.

The page should say that from 26.11.1 searches keep answering during an explicit rebuild, that earlier engines made every search wait, and that a cold index still waits.