Improve the documentation and naming of hook_block_view_alter() hooks

Issue Summary

Problem/Motivation

This is a followup issue for #1535868: Change notice: Convert all blocks into plugins.

• hook_block_view_ID_alter() and hook_block_view_NAME_alter() currently have the same documented example in block.api.php.
• It is unclear from the documentation what ID and NAME represent.

Proposed resolution

• Improve the documentation to clarify the different uses of the two hooks.
• Provide differing, plausible example implementations for each.
• Consider renaming the hooks to use more explicit tokens than ID and NAME.
• Include example hook names in the documentation.