MySQL: update readme; complete initializing a fresh instance paragraph

[gouglhupf]

Completes and clarifies the “Initializing a fresh instance” section in mysql/content.md to accurately document how initialization files are discovered and executed by the image.

What changed

Expanded the paragraph to explicitly document:

• Supported file extensions in "/docker-entrypoint-initdb.d": .sh, .sql, .sql.gz, .sql.bz2, .sql.xz, .sql.zst.
• Shell script handling: .sh without execute permission are sourced; executable .sh are run.

Rationale

The previous text was incomplete/ambiguous about which files run and sourcing vs executing .sh files.
This aligns the docs with the actual behavior in the entrypoint and aligns with the wording used in the mariadb image.

Impact

Docs-only change

[tianon]

Thanks!