Document the arguments form of commands.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@287926 91177308-0d34-0410-b5e6-96231b3b80d8

docs/JSONCompilationDatabase.rst

@@ -78,6 +78,8 @@ The contracts for each field in the command object are:
    Parameters use shell quoting and shell escaping of quotes, with '``"``'
    and '``\``' being the only special characters. Shell expansion is not
    supported.
+-  **arguments:** The compile command executed as list of strings.
+   Either **arguments** or **command** is required.
 
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