A better name for validation related module (`marshmallow.validate`), decorator (`@validates`), and field argument (`validate=`)

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IMO, the current name of validation-related is not intuitive enough. The following API should be changed:

- The `marshamllow.vaildate` module that stores all the validator classes. The `validators` (noun) is used in docs, while the module name is `validate` (verb).
- The decorators used to register a validation method for field or schema: `@validates` and `@validates_schema`. The `s` is quite redundant in my view. Any reasons behind these `validates` names in Marshmallow? is it just because the validate namespace is occupied by the `marshmallow.validate` module?
- The `validate` keyword argument in field classes. The `validate` is a verb, while it used to accept one or more validators.

Proposal:

- Change the module `marshmallow.validate` to `marshmallow.validators`.
- Change the decorator `@validates` to `@validate`.
- Change the decorator `@validates_schema` to `@validate_schema`.
- Change the field argument `validate` to `validator` and `validators`. `validator` accepts one callable; `validators` accepts a list of callable.

Example:

```python
from marshmallow import Schema, validate, validate_schema
from marshmallow.fields import String, Integer
from marshmallow.validators import Length

class PetSchema(Schema):
name = String(required=True, validator=Length(0, 10))
category = String(required=True, validators=[OneOf(['dog', 'cat']), Length(0, 10)])

@validate('name')
def validate_name(...):
pass

@validate_schema
def validate_something(...):
pass
```

Related issues:

- Support validator=, validators=, or validate_with= (#645)
- It seems better changing the validate module to "validators" (#1650)

Originally posted in greyli/apiflask#62

Start by reviewing the marshmallow.validate module, the @validates and @validates_schema decorators, and the field validate argument to map their current public uses. Compare the proposal with related issues #645 and #1650; the work is complete only when the proposed names are applied consistently across the API and its documented examples.

Written by the indexing model from the issue text.