#1625509: describe behaviour of import lock

Doc/library/imp.rst

@@ -114,10 +114,16 @@ This module provides an interface to the mechanisms used to implement the
 
 .. function:: acquire_lock()
 
-   Acquires the interpreter's import lock for the current thread.  This lock should
+   Acquire the interpreter's import lock for the current thread.  This lock should
    be used by import hooks to ensure thread-safety when importing modules. On
    platforms without threads, this function does nothing.
 
+   Once a thread has acquired the import lock, the same thread may acquire it
+   again without blocking; the thread must release it once for each time it has
+   acquired it.
+
+   On platforms without threads, this function does nothing.
+
    .. versionadded:: 2.3