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.

helpers/content.go

@@ -42,6 +42,7 @@ var SummaryLength = 70
 // SummaryDivider denotes where content summarization should end. The default is "<!--more-->".
 var SummaryDivider = []byte("<!--more-->")
 
+// ContentSpec provides functionality to render markdown content.
 type ContentSpec struct {
 	blackfriday                map[string]interface{}
 	footnoteAnchorPrefix       string
@@ -50,6 +51,8 @@ type ContentSpec struct {
 	cfg config.Provider
 }
 
+// NewContentSpec returns a ContentSpec initialized
+// with the appropriate fields from the given config.Provider.
 func NewContentSpec(cfg config.Provider) *ContentSpec {
 	return &ContentSpec{
 		blackfriday:                cfg.GetStringMap("blackfriday"),