docs: Explain behavior change in --tlsv1. options since 7.54

docs: Explain behavior change in --tlsv1. options since 7.54

Since 7.54 --tlsv1. options use the specified version or later, however
older versions of curl documented it as using just the specified version
which may or may not have happened depending on the TLS library.
Document this discrepancy to allay confusion for users familiar with the
old documentation that expect just the specified version.

Fixes https://github.com/curl/curl/issues/4097
Closes https://github.com/curl/curl/pull/4119