tpl: Cleanup strings.TrimPrefix and TrimSuffix

These funcs were added during the move to namespaces but were
undocumented.  This commit fixes the order of the arguments and adds the
funcs to the method mapping.

tpl/strings/init.go

@@ -119,6 +119,14 @@ func init() {
 			},
 		)
 
+		ns.AddMethodMapping(ctx.TrimPrefix,
+			nil,
+			[][2]string{
+				{`{{ "aabbaa" | strings.TrimPrefix "a" }}`, `abbaa`},
+				{`{{ "aabbaa" | strings.TrimPrefix "aa" }}`, `bbaa`},
+			},
+		)
+
 		ns.AddMethodMapping(ctx.TrimRight,
 			nil,
 			[][2]string{
@@ -126,6 +134,14 @@ func init() {
 			},
 		)
 
+		ns.AddMethodMapping(ctx.TrimSuffix,
+			nil,
+			[][2]string{
+				{`{{ "aabbaa" | strings.TrimSuffix "a" }}`, `aabba`},
+				{`{{ "aabbaa" | strings.TrimSuffix "aa" }}`, `aabb`},
+			},
+		)
+
 		ns.AddMethodMapping(ctx.Title,
 			[]string{"title"},
 			[][2]string{