Guide should warn about default_variants

[Ryan Schmidt]

The guide says this about default_variants:

>     If variants are defined, then the default_variants value lists  
> which variants are enabled by default. This allows for Portfile  
> modularity and also allows users to suppress default variants if  
> they wish.
>
>         * Default: none
>         * Example: default_variants +ssl +tcpd
>
>     Default variants may be suppressed by preceding a variant name  
> with a "-" as shown in this example.
>
>     %% port install foo -ssl

A MacPorts bug makes default_variants inadvisable to use for variants  
that you might conceivably want to disable. (If, as in the example  
above, you "port install foo -ssl", foo -ssl is installed. But when  
you later need to upgrade foo, default_variants +ssl will take  
precedence and you'll be left with just "foo" (no "-ssl") installed.)  
Therefore, this section should add a note like this:


"The use of default_variants is discouraged. Instead, it's recommend  
that a port be built such that the most commonly requested  
functionality is on, and if needed, can be disabled with a  
"+no_something" variant. To extend the above example, build the port  
so that ssl and tcpd functionality is on, without needing to select  
any variant. Provide "no_ssl" and "no_tcpd" variants if there's a  
good reason someone might want to disable those features."