Update/add comments explaining the purpose of while(!Serial) in examples

The previous comments (where they existed) only mentioned Leonardo but there are other boards with native USB.

Fixes a regression of an issue previously reported/fixed at arduino/Arduino#3786, and expands it  to the other two Language Reference pages that use while(!Serial) in their examples.

Author: per1234

Language/Functions/Communication/Serial/ifSerial.adoc

@@ -68,7 +68,7 @@ void setup() {
  //Initialize serial and wait for port to open:
   Serial.begin(9600);
   while (!Serial) {
-    ; // wait for serial port to connect. Needed for Leonardo only
+    ; // wait for serial port to connect. Needed for native USB
   }
 }
 

Language/Variables/Data Types/String/Functions/reserve.adoc

@@ -56,7 +56,7 @@ void setup() {
   // initialize serial and wait for port to open:
   Serial.begin(9600);
   while (!Serial) {
-    ; // wait for serial port to connect. Needed for Leonardo only
+    ; // wait for serial port to connect. Needed for native USB
   }
 
   myString.reserve(26);

Language/Variables/Utilities/PROGMEM.adoc

@@ -77,7 +77,7 @@ char myChar;
 
 void setup() {
   Serial.begin(9600);
-  while (!Serial);
+  while (!Serial);  // wait for serial port to connect. Needed for native USB
 
   // put your setup code here, to run once:
   // read back a 2-byte int
@@ -144,7 +144,7 @@ char buffer[30];    // make sure this is large enough for the largest string it
 void setup()
 {
   Serial.begin(9600);
-  while(!Serial);
+  while(!Serial); // wait for serial port to connect. Needed for native USB
   Serial.println("OK");
 }