docs: clarify behavior of default_both_libraries option

Signed-off-by: Daiki Ueno <[email protected]>

docs/markdown/Builtin-options.md

@@ -183,11 +183,16 @@ fails.
 
 #### Details for `default_both_libraries`
 
-Since `1.6.0`, you can select the default type of library selected when using
-a `both_libraries` object. This can be either 'shared' (default value, compatible
-with previous meson versions), 'static', or 'auto'. With auto, the value from
-`default_library` option is used, unless it is 'both', in which case 'shared'
-is used instead.
+Since `1.6.0`, you can specify the default type of library selected when using a
+`both_libraries` object with `default_both_libraries`. Note that, unlike
+`default_library`, this option does not affect how the library artifacts are
+built, but how they are internally linked to the dependent targets within the
+same project.
+
+The possible values of this option are 'shared' (default value, compatible with
+previous meson versions), 'static', and 'auto'. With auto, the value from the
+`default_library` option is used, unless it is 'both', in which case 'shared' is
+used instead.
 
 When `default_both_libraries` is 'auto', passing a [[@both_libs]] dependency
 in [[both_libraries]] will link the static dependency with the static lib,