⎕NA: Improve opening text as the extensions are not definitive

Re: language-reference-guide/system-functions/na/

Currently says:

A library is implemented according to the Operating System as follows:

- a Dynamic Link Library(DLL) under Windows
- a Shared Library (.so or .dylib) under Linux or macOS
- a static library (.a) under AIX

Replace this with:

Code libraries generally (although there can be exceptions) have the following file extensions:

- **.dll** (Dynamic Link Library) under Microsoft Windows
- **.so** (Shared Library) under Linux
- **.dylib** (Dynamic Library) under macOS
- **.a** (static library) or **.so** under AIX