qgis-api-doc software package provides QGIS API documentation, you can install in your Ubuntu 14.04 LTS (Trusty Tahr) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install qgis-api-doc
qgis-api-doc is installed in your system.
Make ensure the qgis-api-doc package were installed using the commands given below,
$ sudo dpkg-query -l | grep qgis-api-doc *
You will get with qgis-api-doc package name, version, architecture and description in a table
| Distro | Ubuntu 14 04 lts trusty tahr |
| Section | Science |
| Package | qgis-api-doc |
| Version | 2.0.1-2build2 |
| Description | QGIS API documentation |
| Sub Section | Science |