Handle plural acronyms in doc_markdown

tests/ui/doc/doc-fixable.fixed

@@ -230,3 +230,8 @@ fn issue_11568() {}
 
 /// There is no try (`do()` or `do_not()`).
 fn parenthesized_word() {}
+
+/// `ABes`
+/// OSes
+/// UXes
+fn plural_acronym_test() {}

tests/ui/doc/doc-fixable.rs

@@ -230,3 +230,8 @@ fn issue_11568() {}
 
 /// There is no try (do() or do_not()).
 fn parenthesized_word() {}
+
+/// ABes
+/// OSes
+/// UXes
+fn plural_acronym_test() {}

tests/ui/doc/doc-fixable.stderr

@@ -341,5 +341,16 @@ help: try
 LL | /// There is no try (do() or `do_not()`).
    |                              ~~~~~~~~~~
 
-error: aborting due to 31 previous errors
+error: item in documentation is missing backticks
+  --> tests/ui/doc/doc-fixable.rs:234:5
+   |
+LL | /// ABes
+   |     ^^^^
+   |
+help: try
+   |
+LL | /// `ABes`
+   |     ~~~~~~
+
+error: aborting due to 32 previous errors