Fix weak symbol linkage in SBStructuredData, update docs.

Summary:
This change fixes an issue where I was leaking a weakly-linked symbol in
the SBAPI. It also updates the docs to call out what I did wrong.

Fixes:
rdar://28882483

Reviewers: jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D26470

llvm-svn: 286413