Improve comments for custom field length check

Function custom_field_get_id_from_name() contains code to check for
truncated custom fields names. This dates back to when the history
table's field_name column was only 32 chars long, whereas the custom
field's size is 64.

Even though the history's field_name size has been increased to 64 since
1.1.0a4 (see issue 0008002), the check must be maintained to ensure
compatibility with upgraded legacy systems.

The fix ce3450aaabbd9d9feceb03d77d4358f195e7d966 implemented in master
branch cannot be applied here as it contains a schema update. Instead,
comments have been reworded and obsolete reference to old schema
removed.

Fixes 0014650