Source code documentation is an important process for software project maintenance and management. The documentation process always consumes a lot of time and effort from human experts. The software project document should be concise and clear without any ambiguity. Existing source code documentation tools like JavaDoc are very limited in the market. Also, tools can identify only the predefined methods of corresponding programming languages. In this paper, a novel system is proposed to automate the source code documentation process for C programming language using the source code summarization technique of NLP. The core component of this proposed system i.e., Software Word Usage Model (SWUM) build using Context-Free Grammars and NLP preprocessing techniques. This system can successfully generate the documentation for a C program along with predefined and user-defined methods using Natural Language Generation technique. This proposed system can document a program in two major formats; method-based, abstract level and statement-based, detailed level. The proposed system efficiency is evaluated by comparing system-generated source code documentation with an expert generated documentation. Results obtained from that comparison shows that the proposed system can give better performance for small and medium-size software projects.