Document Explorer Help* introduces us to search for the phrases:

Quotation Marks and Parentheses

Use quotation marks or parentheses to group terms, phrases, and operators together. For example, "new operator" (including the quotation marks) returns topics that contain the literal phrase "new operator" and all its grammatical variations. Without the quotation marks, the query is equivalent to specifying a new AND operator, which returns topics that contain both of the individual words, instead of the phrase.

It’s only partially true. You can’t search phrase “AT command” because search engine ignore quotations marks.

*Microsoft Document Explorer 7.0 is a part of Microsoft TechNet International Library (November 2006).