mdbx: rework API and Docs around Handle-Slow-Readers (no algorithmic changes).

Change-Id: I5b76a8400ce6f5f241f8e4a7f53d746fe39f8e1e

docs/_restrictions.md

@@ -147,9 +147,9 @@ or debugging of a client application while retaining an active read
 transaction. LMDB this results in `MDB_MAP_FULL` error and subsequent write
 performance degradation.
 
-MDBX mostly solve "long-lived" readers issue by the lack-of-space callback
-which allow to aborts long readers, and by the `MDBX_LIFORECLAIM` mode which
-addresses subsequent performance degradation.
+MDBX mostly solve "long-lived" readers issue by using the Handle-Slow-Readers
+\ref MDBX_hsr_func callback which allows to abort long-lived read transactions,
+and using the \ref MDBX_LIFORECLAIM mode which addresses subsequent performance degradation.
 The "next" version of libmdbx (MithrilDB) will completely solve this.
 
 - Avoid suspending a process with active transactions. These would then be

docs/_starting.md

@@ -236,6 +236,7 @@ The full \ref c_api documentation lists further details below, like how to:
 - Sstimate size of range query result: \ref c_rqest.
 - Double performance by LIFO reclaiming on storages with write-back: \ref MDBX_LIFORECLAIM.
 - Use sequences and canary markers: \ref mdbx_dbi_sequence(), \ref MDBX_canary.
-- Use lack-of-space callback (aka OOM-KICK): \ref mdbx_env_set_oomfunc().
+- Use Handle-Slow-Readers callback to resolve a database full/overflow issues
+  due to long-lived read transactions: \ref mdbx_env_set_hsr().
 - Use exclusive mode: \ref MDBX_EXCLUSIVE.
 - Define custom sort orders (but this is recommended to be avoided).