Reword "computationally heavy" language in docs

[sffc]

Our docs have verbiage such as:

For that reason, one should think of the process of formatting a date in two steps - first, a computational heavy construction of DateTimeFormatter, and then fast formatting of DateTime data using the instance."

This is somewhat misleading. We should come up with better verbiage.

[zbraniecki]

Do we have any measurements of the actual cost of DTF construction?

[sffc]

Let's block this on #3865 and write the verbiage based on measurements after those changes land.

[Manishearth]

Assigning to @sffc since it's a part of the whole set of datetime changes