Merge pull request #3061 from WalterBright/graphemeStride-doc

improve doc for std.uni.graphemeStride()

std/uni.d

@@ -6421,10 +6421,18 @@ template genericDecodeGrapheme(bool getValue)
 public: // Public API continues
 
 /++
-    Returns the length of grapheme cluster starting at $(D index).
+    Computes the length of grapheme cluster starting at $(D index).
     Both the resulting length and the $(D index) are measured
     in $(S_LINK Code unit, code units).
 
+    Params:
+        C = type that is implicitly convertible to $(D dchars)
+        input = array of grapheme clusters
+        index = starting index into $(D input[])
+
+    Returns:
+        length of grapheme cluster
+
     Example:
     ---
     // ASCII as usual is 1 code unit, 1 code point etc.