Add link to Knowledge Base article when flood limit's reached

Problem/Motivation

There is an Acquia Knowledge Base article about flood control mechanism here. This explains what happens when the query limits have been reached. (e.g. The 'Flood protection has blocked request of type select.' is logged, and/or a message on the screen appears. This is thrown by the Flood helper, or the SearchSubscriber). The helper/subscriber however does not link to this article confusing site owners about the error - not knowing the config value behind this error can be changed and is there to protect from bot/DOS attacks or high value traffics targeting those queries.

Steps to reproduce

Use high traffic on search pages where acquia_search is used, and reach the limit of flood control set.

Proposed resolution

Update the error message in Flood and in SearchSubscriber to involve the link to the article.

Remaining tasks

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User interface changes

When the limit is reached, the message displayed on the screen changes, and includes a link to the KB article. Also, the error message logged has the link to the KB article.

API changes

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Data model changes

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