documentation: add small explanation for commit signing

Author: implausible

include/git2/commit.h

@@ -507,7 +507,13 @@ GIT_EXTERN(int) git_commit_dup(git_commit **out, git_commit *source);
  *
  * The callback will be called with the commit content, giving a user an
  * opportunity to sign the commit content. The signature_field
- * buf may be left empty to specify the default field.
+ * buf may be left empty to specify the default field "gpgsig".
+ *
+ * Signatures can take the form of any string, and can be created on an arbitrary
+ * header field. Signatures are most commonly used for verifying authorship of a
+ * commit using GPG or a similar cryptographically secure signing algorithm.
+ * See https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work for more
+ * details.
  *
  * When the callback:
  * - returns GIT_PASSTHROUGH, no signature will be added to the commit.