MicroRNA: get rid of FIXMEs from class documentation

1. List of arguments passed to the constructor now references the
superclass (a'la Feature class hierarchy) instead of listing them
explicitly.

2. Arm documentation now correctly lists values this property can take.

modules/Bio/EnsEMBL/MicroRNA.pm

@@ -71,31 +71,18 @@ use parent qw(Bio::EnsEMBL::RNAProduct);
 
 =head2 new
 
-  Arg [-SEQ_START]    : The offset in the Translation indicating the start
-                        position of the product sequence.
-  Arg [-SEQ_END]      : The offset in the Translation indicating the end
-                        position of the product sequence.
-  Arg [-STABLE_ID]    : The stable identifier for this RNAPRoduct
-  Arg [-VERSION]      : The version of the stable identifier
-  Arg [-DBID]         : The internal identifier of this MicroRNA
-  Arg [-ADAPTOR]      : The TranslationAdaptor for this MicroRNA
-  Arg [-SEQ]          : Manually sets the nucleotide sequence of this
-                        rnaproduct. May be useful if this rnaproduct is not
-                        stored in a database.
-  Arg [-CREATED_DATE] : the date the rnaproduct was created
-  Arg [-MODIFIED_DATE]: the date the rnaproduct was modified
-  # FIXME: does all of the above have to be repeated here or can we somehow pull this from the superclass?
   Arg: [-ARM]         : which arm of the hairpin precursor this miRNA comes
-                        from. Negative values indicate 3', positive ones - 5'.
-                        FIXME: is this right?
+                        from. Returns 3 and 5 for 3' and 5', respectively.
+  Arg [...]           : Named arguments to superclass constructor
+                        (see Bio::EnsEMBL::RNAProduct)
   Example    : my $miR = Bio::EnsEMBL::MicroRNA->new(
                  -SEQ_START => 36,
                  -SEQ_END   => 58,
-                 -ARM       => -1
+                 -ARM       => 3
                );
   Description: Constructor.  Creates a new MicroRNA object
   Returntype : Bio::EnsEMBL::MicroRNA
-  Exceptions : none
+  Exceptions : throw if ARM value is out of bounds
   Caller     : general
   Status     : In Development
 
@@ -123,12 +110,16 @@ sub new { ## no critic (Subroutines::RequireArgUnpacking)
 
 =head2 arm
 
+    Arg [1]     : (optional) int $arm which arm of the hairpin precursor
+                  this miRNA comes from
     Example     : $mirna_arm = $mirna->arm();
-    Description : Returns the arm of the hairpin this miRNA comes from.
-                  Negative values indicate the 3' end, positive ones
-                  - the 5' one. FIXME: is this right?
+                  $mirna->arm(3);
+    Description : Sets or returns the arm of the hairpin this miRNA comes
+                  from. Accepted values are 3 and 5 for 3' and 5',
+                  respectively.
     Return type : Integer
-    Exceptions  : warn if multiple 'mirna_arm' attributes exist
+    Exceptions  : throw if setter is passed an incorrect value;
+                  warn if multiple 'mirna_arm' attributes exist.
     Caller      : General
     Status      : Stable