Merge #593

593: Docs for completion r=matklad a=kjeremy

The first commit adds documentation support to CompletionItems.

The second one I am unsure about. Is that the right way to add docs for functions? If so should I do something similar for other `hir` types and CompletionItems?

Co-authored-by: Jeremy Kolb <kjeremy@gmail.com>
Co-authored-by: Jeremy A. Kolb <jkolb@ara.com>

crates/ra_ide_api_light/src/snapshots/tests__file_structure.snap

@@ -1,7 +1,9 @@
-Created: 2019-01-15T11:15:21.073862814+00:00
-Creator: insta@0.1.4
-Source: crates/ra_ide_api_light/src/structure.rs
-
+---
+created: "2019-01-22T14:45:01.959724300+00:00"
+creator: insta@0.4.0
+expression: structure
+source: "crates\\ra_ide_api_light\\src\\structure.rs"
+---
 [
     StructureNode {
         parent: None,

crates/ra_ide_api_light/src/snapshots/tests__highlighting.snap

@@ -1,7 +1,9 @@
-Created: 2019-01-15T11:15:21.073858657+00:00
-Creator: insta@0.1.4
-Source: crates/ra_ide_api_light/src/lib.rs
-
+---
+created: "2019-01-22T14:45:01.959724300+00:00"
+creator: insta@0.4.0
+expression: hls
+source: "crates\\ra_ide_api_light\\src\\lib.rs"
+---
 [
     HighlightedRange {
         range: [1; 11),