Pod API documentation refers to `C_IDENTIFIER`

> Name of the environment variable. Must be a C_IDENTIFIER.

> The keys defined within a source must be a C_IDENTIFIER.

> Must be a C_IDENTIFIER.

What is a "C_IDENTIFIER"? This doesn't appear to be defined anywhere in the Kubernetes documentation.

Search the Kubernetes documentation for the three quoted C_IDENTIFIER references in the Pod API documentation and inspect the surrounding text. Define or link the term consistently at each occurrence, then verify that readers can understand the required identifier format from the updated documentation.

Written by the indexing model from the issue text.