doc(rdb): add details for rdb instance settings option

[Codelax]

Fix #791

[codecov-commenter]

Codecov Report

Merging #1305 (18581af) into master (534d1d2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1305   +/-   ##
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  Coverage   69.88%   69.88%           
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  Files         102      102           
  Lines       16332    16332           
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  Hits        11414    11414           
  Misses       3934     3934           
  Partials      984      984           

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[Codelax]

I could also add an example in documentation but options may be deprecated in the future and this is just an hcl map

resource scaleway_rdb_instance main {
  name = "test-rdb"
  node_type = "db-dev-s"
  disable_backup = true
  engine = "PostgreSQL-11"
  user_name = "my_initial_user"
  password = "thiZ_is_v&ry_s3cret"
  settings = {
    work_mem = "4"
    max_connections = "200"
    effective_cache_size = "1300"
    maintenance_work_mem = "150"
    max_parallel_workers = "2"
    max_parallel_workers_per_gather = "2"
  }
}