Add acts-as-taggable-on and polymorphic searches to documentation

docs/docs/going-further/acts-as-taggable-on.md

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+---
+title: Acts-as-taggable-on
+sidebar_position: 13
+---
+
+If you have an `ActiveRecord` model and you're using [acts-as-taggable-on](https://github.com/mbleigh/acts-as-taggable-on),
+chances are you might want to search on tagged fields.
+
+Suppose you have this model:
+
+```rb
+class Task < ApplicationRecord
+  acts_as_taggable_on :projects
+end
+```
+
+and you have the following two instances of `Task`:
+
+```rb
+{ id: 1, name: 'Clean up my room',        projects: [ 'Home', 'Personal' ] }
+{ id: 2, name: 'Complete math exercises', projects: [ 'Homework', 'Study' ] }
+```
+
+When you're writing a `Ransack` search form, you can choose any of the following options:
+
+```erb
+<%= search_form_for @search do |f| %>
+  <%= f.text_field :projects_name_in   %> <!-- option a -->
+  <%= f.text_field :projects_name_eq   %> <!-- option b -->
+  <%= f.text_field :projects_name_cont %> <!-- option c -->
+<% end %>
+```
+
+### Option a - match keys exactly
+
+Option `a` will match keys exactly. This is the solution to choose if you want to distinguish 'Home' from 'Homework': searching for 'Home' will return just the `Task` with id 1. It also allows searching for more than one tag at once (comma separated):
+- `Home, Personal` will return task 1
+- `Home, Homework` will return task 1 and 2
+
+### Option b - match key combinations
+
+Option `b` will match all keys exactly. This is the solution if you wanna search for specific combinations of tags:
+- `Home` will return nothing, as there is no Task with just the `Home` tag
+- `Home, Personal` will return task 1
+
+### Option c - match substrings
+
+Option `c` is used to match substrings. This is useful when you don't care for the exact tag, but only for part of it:
+- `Home` will return task 1 and 2 (`/Home/` matches both `"Home"` and `"Homework"`)

docs/docs/going-further/polymorphic-search.md

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+---
+title: Polymorphic Searches
+sidebar_position: 14
+---
+
+When making searches from polymorphic models it is necessary to specify the type of model you are searching. 
+
+For example:
+
+Given two models
+
+```ruby
+class House < ActiveRecord::Base
+  has_one :location, as: :locatable
+end
+
+class Location < ActiveRecord::Base
+  belongs_to :locatable, polymorphic: true
+end
+```
+
+Normally (without polymorphic relationship) you would be able to search as per below:
+
+```ruby
+Location.ransack(locatable_number_eq: 100).result
+```
+
+However when this is searched you will get the following error
+
+```ruby
+ActiveRecord::EagerLoadPolymorphicError: Can not eagerly load the polymorphic association :locatable
+```
+
+In order to search for locations by house number when the relationship is polymorphic you have to specify the type of records you will be searching and construct your search as below:
+
+```ruby
+Location.ransack(locatable_of_House_type_number_eq: 100).result
+```
+
+note the `_of_House_type_` added to the search key. This allows Ransack to correctly specify the table names in SQL join queries.