Renames hideBlockByCapability for clarity

[guarani]

This PR renames hideBlockByCapability to showBlockByCapability since, if a block is passed into this method along with a true argument, the function's logic is to display the block (not hide it).

To test:

Run locally and verify if Jetpack blocks are still shown correctly according to their availability and their corresponding editor capability's value.

PR submission checklist:

• [x] I have considered adding unit tests where possible.
• [x] I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.