Began individual function docs

docs/api/srfi/143.md

@@ -55,14 +55,34 @@ See the [SRFI document](http://srfi.schemers.org/srfi-143/srfi-143.html) for mor
 
 # fx-width 
 
+    (fx-width)
+
+Bound to the value `w` that specifies the implementation-defined fixnum integer range.
+
 # fx-greatest 
 
+    (fx-greatest)
+
+Returns value of the largest representable fixnum.
+
 # fx-least
 
+    (fx-least)
+
+Returns value of the smallest representable fixnum.
+
 # fixnum?
 
+    (fixnum? obj)
+
+Returns `#t` if `obj` is an exact integer within the fixnum range, and `#f` otherwise.
+
 # fx=? 
 
+    (fx=? i j)
+
+Semantically equivalent to `=`.
+
 # fx<? 
 
 # fx>?