kubectl: rewrite docstrings in several files

Fixing inaccuracies and clarifying in the case of ambiguities.

pkg/kubectl/kubectl.go

@@ -212,7 +212,10 @@ func parseFileSource(source string) (keyName, filePath string, err error) {
 	}
 }
 
-// parseLiteralSource parses the source key=val pair
+// parseLiteralSource parses the source key=val pair into its component pieces.
+// This functionality is distinguished from strings.SplitN(source, "=", 2) since
+// it returns an error in the case of empty keys, values, or a missing equals
+// sign.
 func parseLiteralSource(source string) (keyName, value string, err error) {
 	// leading equal is invalid
 	if strings.Index(source, "=") == 0 {