In general terms, documentation is any communicable material (such as text, video, audio, etc., or combinations thereof) used to explain some attributes of an object, system or procedure. It is often used to mean engineering or software documentation, which is usually paper books or computer readable files (such as HTML pages) that describe the structure and components, or on the other hand, operation, of a system/product.
A professional whose field and work is more or less exclusively to write documentation is called a documenter. Normally, documenters are trained or have a background in technical writing, along with some knowledge of the subject(s) they are documenting. Often, though, some part or all of the documentation process is done by the engineers responsible for the system/product to be documented.
By engineers, perhaps among software engineers in particular, documentation is often referred to as the "boring side" of engineering, or considered a necessary evil. This is largely unavoidable since most engineers prefer building things to documenting them, and being implicit experts in what they have built, they may have little motivation in documenting their creations so that others may understand them.
Acorn Object Format v2 - Historical document defining a file format called ARM Object Format, which is used by language processors for ARM-based systems.
Acorn Object Library Format v1 - Historical document defining a file format called ARM Object Library Format, which is used by language processors for ARM-based systems.
Acorn Technical Documents Archive - Technical documents, including many early ones. A hardware bias, includes links to hardware projects.