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/path.go

@@ -32,6 +32,7 @@ var (
 	// ErrThemeUndefined is returned when a theme has not be defined by the user.
 	ErrThemeUndefined = errors.New("no theme set")
 
+	// ErrWalkRootTooShort is returned when the root specified for a file walk is shorter than 4 characters.
 	ErrWalkRootTooShort = errors.New("Path too short. Stop walking.")
 )
 
@@ -480,7 +481,7 @@ func FindCWD() (string, error) {
 
 // SymbolicWalk is like filepath.Walk, but it supports the root being a
 // symbolic link. It will still not follow symbolic links deeper down in
-// the file structure
+// the file structure.
 func SymbolicWalk(fs afero.Fs, root string, walker filepath.WalkFunc) error {
 
 	// Sanity check