Fix mpc arg documentation

We do not support a comma-delimited list. Supporting it wouldn't be significantly more efficient than having the caller invoke the tool multiple times, so I'm fixing the docs to reflect functionality rather than adding list support.

Author: AArnott

src/MessagePack.Generator/MessagepackCompiler.cs

@@ -39,7 +39,7 @@ await Host.CreateDefaultBuilder()
         }
 
         public async Task RunAsync(
-            [Option("i", "Input path of analyze MSBuild project file or directory, if input multiple project files split with ','.")] string input,
+            [Option("i", "Input path to MSBuild project file or the directory containing Unity source files.")] string input,
             [Option("o", "Output file path(.cs) or directory (multiple generate file).")] string output,
             [Option("c", "Conditional compiler symbols, split with ','. Ignored if a project file is specified for input.")] string? conditionalSymbol = null,
             [Option("r", "Set resolver name.")] string resolverName = "GeneratedResolver",