Guide 3.2.1: Say that variant specification on port install does not overrule existing ports

[JDLH (Jim DeLaHunt)]

The ​MacPorts Guide, section 3.2.1. Invoking Variants does not describe well how port install propagates variant invocations to dependencies. It should explain a) how it propagates variant invocations to not-installed dependencies, b) how it does not overrule variants on installed dependencies, and c) mention of the --enforce-dependencies option.

The Guide presently says, "This [MacPorts not warning about misspelled variant names] happens because MacPorts will also use the specified variants for any dependencies." That is an oblique reference to port install propagating variants. The explanation should be clearer, and include all three elements above.

This behaviour is discussed in a ​MacPorts-user list thread "Should I expect a +quartz variant to propagate to dependencies, and overrule existing variants?" from June 2022. It may have helpful information for a writer.