Clarify metadata documentation

While portal-wide metadata includes samples with bulk libraries, the `seq_unit` column in the actual portal metadata (and other similar metadata columns) only provide details about the single-cell/nuclei or spot sequencing.

However, our documentation indicates that `bulk` may be a possible value, which it currently is not: https://github.com/AlexsLemonade/scpca-docs/blob/765c86fcef085d944bfafe4b51389e2492d0b112/docs/download_files.md?plain=1#L120

We should update this documentation to clarify which _libraries_ are in this file more generally.

Start at docs/download_files.md around line 120 and review the metadata description against the issue's distinction between portal-wide libraries and single-cell, nuclei, or spot sequencing. Update the wording so it clearly explains which libraries are represented and does not imply that bulk is a current seq_unit value; confirm the surrounding documentation remains consistent.

Written by the indexing model from the issue text.