Refine documentation of default name_prefix

Refine documentation of the default name_prefix, so people don't get the
impression they can write logic which uses 'lib' as the default, when
they should be defaulting to '[]', to let us take care of the
complexities.

docs/markdown/Reference-manual.md

@@ -1191,9 +1191,9 @@ The keyword arguments for this are the same as for
 
 - `name_prefix` the string that will be used as the prefix for the
   target output filename by overriding the default (only used for
-  libraries). By default this is `lib` on all platforms and compilers
+  libraries). By default this is `lib` on all platforms and compilers,
-  except with MSVC shared libraries where it is omitted to follow
+  except for MSVC shared libraries where it is omitted to follow
-  convention.
+  convention, and Cygwin shared libraries where it is `cyg`.
 - `name_suffix` the string that will be used as the suffix for the
   target output filename by overriding the default (see also:
   [executable()](#executable)). By default, for shared libraries this
@@ -1201,7 +1201,7 @@ The keyword arguments for this are the same as for
   For static libraries, it is `a` everywhere. By convention MSVC
   static libraries use the `lib` suffix, but we use `a` to avoid a
   potential name clash with shared libraries which also generate
-  `xxx.lib` import files.
+  import libraries with a `lib` suffix.
 - `rust_crate_type` specifies the crate type for Rust
   libraries. Defaults to `dylib` for shared libraries and `rlib` for
   static libraries.