[label] Document meaning of kNear and kFar distances

The intel manual knows "short jumps", "near jumps" and "far jumps",
where our Label::kNear corresponds to short jumps and Label::kFar
corresponds to near jumps.
Add some words to document this.

R=mstarzinger@chromium.org

Change-Id: I2b41cc770ee2c1fecff5e7363cede264ca3cd033
Reviewed-on: https://chromium-review.googlesource.com/824270Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50075}

src/label.h

@@ -18,7 +18,10 @@ namespace internal {
 
 class Label {
  public:
-  enum Distance { kNear, kFar };
+  enum Distance {
+    kNear,  // near jump: 8 bit displacement (signed)
+    kFar    // far jump: 32 bit displacement (signed)
+  };
 
   Label() = default;