Fix section on recommnedations for number of custom fields

Need to check on the following as para 1 talks says "limit the number of custom fields in a set to fewer than 40" but para 2 say "creating more than 40 custom data sets may result in..."

so is it custom fields or custom data sets that need to be no more than or both.

Custom field set limitations

Bear the following in mind when creating custom fields:

Limit the number of custom fields in a set to fewer than 40. If you have approached this total, we recommend you import some records and test operations like search and export before finalizing your database design and putting it into production.

Why? CiviCRM stores each custom field set as a MySQL table with a column for each custom field in that set. It is important to consider this as you plan your custom data 'schema', as it will have the same advantages and limitations as other MySQL tables. If you put too many fields in one set, you risk exceeding MySQL's limit for maximum row-size, which may present search and data export issues later on (creating more than 40 custom data sets may result in exceeding MySQL's maximum allowed number of left joins - 64 - when performing operations like exporting data). Consult the MySQL Reference Manual for more details."