Update IMPORT statement doc to include PGCOPY

[lnhsingh]

Lauren Hirata Singh (lnhsingh) commented:

It's rare that you'd have PGCOPY data outside of PGDUMP but we support it since we needed to support PGDUMP. However, we have PGCOPY listed in our \h IMPORT text (and mention it in Import Perf Best Practices), so we might want to document it as an option to be consistent.

Note: PGDUMP can read a full SQL file. PGCOPY can only read a single table in COPY format

Jira Issue: DOC-647

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