Clarify the use of -Doption and alternatives [skip ci]

The -Doption=value style argument works for all builtin options,
and compiler options passed to meson. And some universal options,
have additional ways to be passed. However, base options (and compiler
options) do not have exactly the same ways of passing as universal
options.

This change adds a few pieces of information, which might get lost,
if the manual is not read serially.

[skip ci]

docs/markdown/Builtin-options.md

@@ -11,9 +11,10 @@ universal options, base options, compiler options.
 ## Universal options
 
 A list of these options can be found by running `meson --help`. All
-these can be set by passing to `meson` (aka `meson setup`) in any of
-these ways: `--option=value`, `--option value`, `-Doption=value`, or
+these can be set by passing `-Doption=value` to `meson` (aka `meson setup`), or
 by setting them inside `default_options` of `project()` in your `meson.build`.
+Some options can also be set by `--option=value`, or `--option value`--- a list
+is shown by running `meson setup --help`.
 
 For legacy reasons `--warnlevel` is the cli argument for the `warning_level` option.
 
@@ -101,7 +102,9 @@ All other combinations of `debug` and `optimization` set `buildtype` to `'custom
 
 ## Base options
 
-These are set in the same way as universal options, but cannot be shown in the
+These are set in the same way as universal options, either by `-Doption=value`, 
+or by setting them inside `default_options` of `project()` in your `meson.build`.
+However, they cannot be shown in the
 output of `meson --help` because they depend on both the current platform and
 the compiler that will be selected. The only way to see them is to setup
 a builddir and then run `meson configure` on it with no options.
@@ -155,7 +158,8 @@ embedded because `-Wl,-bitcode_bundle` is incompatible with both `-bundle` and
 
 Same caveats as base options above.
 
-The following options are available. Note that both the options themselves and
+The following options are available. They can be set by passing `-Doption=value`
+to `meson`. Note that both the options themselves and
 the possible values they can take will depend on the target platform or
 compiler being used: