Improve 'checkuser-massblock-text' en.json wording

Why:

• The 'checkuser-massblock-text' message needs improvement to make it clearer and easier for translators to translate from English.

What:

• Improve the wording of the 'checkuser-massblock-text' message in en.json

Bug: T366496
Change-Id: Ic8f7fdd86d666d4af121e1b4b0a6928c4f1428d1