Fix a few minor nits in protobuf definitions

[jtraglia]

What type of PR is this?

Other

What does this PR do? Why is it needed?

• Add blank lines between fields with comments. This is for consistency with other files.
• Use blob.size rather than 131072, might as well since it's defined.
• Replace confusing comment for Blob. I get what it's saying but it's strange.
  • BLS_FIELD_ELEMENT should be BYTES_PER_FIELD_ELEMENT.
  • "multiplies" should have been "multiples of".
  • But even then it doesn't read very well.
• Comment for commitments should be MAX_BLOB_COMMITMENTS_PER_BLOCK instead of MAX_BLOBS_PER_BLOCK.
• Fix comment that says SignedBLSToExecutionChange is new to Deneb 😃
• Add comment describing how blob.size is calculated.

[prestonvanloon]

Nice, thanks!