Add semaphore doc text

Author: pitr-ch

lib/concurrent/atomic/semaphore.rb

@@ -153,6 +153,12 @@ def try_acquire_timed(permits, timeout)
   if RUBY_PLATFORM == 'java'
 
     # @!macro semaphore
+    #     
+    #   A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each {#acquire} blocks if necessary
+    #   until a permit is available, and then takes it. Each {#release} adds a permit,
+    #   potentially releasing a blocking acquirer.
+    #   However, no actual permit objects are used; the Semaphore just keeps a count of the number available and
+    #   acts accordingly.
     class JavaSemaphore
       # @!macro semaphore_method_initialize
       def initialize(count)