Auto merge of #56824 - euclio:internal-apis, r=QuietMisdreavus

rustdoc: display rustc_private APIs as "Internal"

This PR updates the display of `rustc_private` APIs to be "Internal" instead of "Experimental", and changes the colors appropriately. It also updates the copy of the `rustc_private` feature to sound more informative and less like a compiler suggestion.

The PR additionally contains a significant refactor of the `short_stability` function to remove duplication and fix a few rendering bugs due to extra or missing spaces.

Before:

![screen shot 2018-12-14 at 11 45 28 am](https://user-images.githubusercontent.com/1372438/50015926-c9768d80-ff95-11e8-9649-5df29df6909b.png)

After:

![screen shot 2018-12-14 at 11 45 11 am](https://user-images.githubusercontent.com/1372438/50015934-cf6c6e80-ff95-11e8-912b-74b893f55425.png)

1 files changed, 1 insertions, 1 deletions

diff --git a/src/librustdoc/html/static/rustdoc.css b/src/librustdoc/html/static/rustdoc.css
index 7a3682503f7..3958986e492 100644
--- a/src/librustdoc/html/static/rustdoc.css
+++ b/src/librustdoc/html/static/rustdoc.css
@@ -769,7 +769,7 @@ body.blur > :not(#help) {
 	display: list-item;
 }
 
-.stab .microscope {
+.stab .emoji {
 	font-size: 1.5em;
 }