rustdoc: add comment to confusing CSS `main { min-width: 0 }`

This CSS was added in cad0fce2053d52b7ba04c458f4c124c8b5c6141e, but the
reason why it was necessary was unclear. This comment makes it clear.

diff --git a/src/librustdoc/html/static/css/rustdoc.css b/src/librustdoc/html/static/css/rustdoc.css
index cf5592da43205d81a9b61907be8ccf7e935192df..5c9468392425a97956351047c2b6deac8a8976f3 100644 (file)
--- a/src/librustdoc/html/static/css/rustdoc.css
+++ b/src/librustdoc/html/static/css/rustdoc.css
@@ -314,7 +314,7 @@ main {
        position: relative;
        flex-grow: 1;
        padding: 10px 15px 40px 45px;
-       min-width: 0;
+       min-width: 0; /* avoid growing beyond the size limit */
 }
 
 .source main {