Mention NetFx40_LegacySecurityPolicy for PublicSign (#99184)

* Update public-signing.md

* Specify "may" because we've seen it work on some machines but not others.

It's not clear to use what the difference is other than a hunch that it fails on Windows Server but works fine on Windows Client OS.

* Add missing angle bracket

docs/project/public-signing.md

@@ -17,6 +17,7 @@ Known issues when debugging and testing public signed assemblies on .NET Framewo
 - You will not be able to load the assembly in an AppDomain where shadow copying is turned on.
 - You will not be able to load the assembly in a partially trusted AppDomain
 - You will not be able to pre-compile ASP.NET applications
+- You may not be able to load the assembly if the `app.config` file specifies `<configuration>`/`<runtime>`/`<NetFx40_LegacySecurityPolicy enabled="true" />`
 
 The `corflags.exe` tool that ships with the .NET Framework SDK can show whether a binary is delay-signed or strong-named. For a delay-signed assembly it may show: