Env::read_txn document possibles errors.

Document error according to mdb_txn_begin documentation.

heed/src/env.rs

@@ -600,6 +600,14 @@ impl Env {
     }
 
     /// Create a transaction with read-only access for use with the environment.
+    ///
+    /// ## Errors
+    ///
+    /// * [heed::mdb::lmdb_error::Error::Panic]: A fatal error occured ealier and the environnement must be shut down
+    /// * [heed::mdb::lmdb_error::Error::MapResized]: Another process wrote data beyond this [Env] mapsize and this env
+    /// map must be resized
+    /// * [heed::mdb::lmdb_error::Error::ReadersFull]: a read-only transaction was requested and the reader lock table is
+    /// full
     pub fn read_txn(&self) -> Result<RoTxn> {
         RoTxn::new(self)
     }