[clang] [docs][clang] refactor diagnostic spec instruction and grammar nit (PR #213432)

[Ambrose Leeb via cfe-commits]

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@@ -3612,7 +3612,8 @@ int A = B; // expected-error {{use of undeclared identifier 'B'}}
 ```
 
 You can place as many diagnostics on one line as you wish. To make the code
-more readable, you can use slash-newline to separate out the diagnostics.
+more readable, you can use back-slash to separate out the diagnostics into
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Sirraide wrote:

I suppose it should be clear from context yes

https://github.com/llvm/llvm-project/pull/213432