Clarify blob sidecars broadcast section

[bharath-123]

In Glamsterdam, validators who self-build (i.e are both the validator and the builder for the slot) still have to broadcast the BlobSidecars by themselves. The spec today mentions that validators do not need to broadcast blob sidecars which feels a bit misleading as validators would still have to broadcast the blob sidecars if they self-build.

Also post-Fulu, validators will broadcast DataColumnSidecars instead of BlobSidecars.

[hangleang]

If the validator choose to self-build, then they are also a builder, therefore they have to act in accordance with the honest builder spec. I see no points to specify this in validator spec. so I think the current spec is clear enough, except the blob/data column sidecars confusion.