Add some missing doc comments

As pointed out by the linter, some exported functions and types are
missing doc comments.
The linter warnings have been reduced from 194 to 116.
Not all missing comments have been added in this commit though.

tpl/math/math.go

@@ -29,14 +29,17 @@ func New() *Namespace {
 // Namespace provides template functions for the "math" namespace.
 type Namespace struct{}
 
+// Add adds two numbers.
 func (ns *Namespace) Add(a, b interface{}) (interface{}, error) {
 	return DoArithmetic(a, b, '+')
 }
 
+// Div divides two numbers.
 func (ns *Namespace) Div(a, b interface{}) (interface{}, error) {
 	return DoArithmetic(a, b, '/')
 }
 
+// Log returns the natural logarithm of a number.
 func (ns *Namespace) Log(a interface{}) (float64, error) {
 	af, err := cast.ToFloat64E(a)
 
@@ -84,10 +87,12 @@ func (ns *Namespace) ModBool(a, b interface{}) (bool, error) {
 	return res == int64(0), nil
 }
 
+// Mul multiplies two numbers.
 func (ns *Namespace) Mul(a, b interface{}) (interface{}, error) {
 	return DoArithmetic(a, b, '*')
 }
 
+// Sub substracts two numbers.
 func (ns *Namespace) Sub(a, b interface{}) (interface{}, error) {
 	return DoArithmetic(a, b, '-')
 }