doc/developer: Clarify symbol naming prefixes section.

doc/developer.texi

@@ -188,13 +188,16 @@ There are the following conventions for naming variables and functions:
 @item
 For local variables no prefix is required.
 @item
-For variables and functions declared as @code{static} no prefix is required.
+For file-scope variables and functions declared as @code{static}, no prefix
+is required.
 @item
-For variables and functions used internally by a library an @code{ff_}
-prefix should be used, e.g. @samp{ff_w64_demuxer}.
+For variables and functions visible outside of file scope, but only used
+internally by a library, an @code{ff_} prefix should be used,
+e.g. @samp{ff_w64_demuxer}.
 @item
-For variables and functions used internally across multiple libraries, use
-@code{avpriv_}. For example, @samp{avpriv_aac_parse_header}.
+For variables and functions visible outside of file scope, used internally
+across multiple libraries, use @code{avpriv_} as prefix, for example,
+@samp{avpriv_aac_parse_header}.
 @item
 For externally visible symbols, each library has its own prefix. Check
 the existing code and choose names accordingly.