Speed up javadoc formatting

Initializing a StringBuilder with a string results in it being
sized to contain only that string -- hence forcing a reallocation
on next append.

This patch updates javadoc formatting code to use a default allocation
and append the initial string afterwards -- leaving some room for
further appends.

Change-Id: I910a3c84d37f462d99815f5ea63700cc78dfd2cc
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>