gstring: fully document semantics of @len for g_string_insert_len

The g_string_insert_len method accepts '-1' for its len parameter,
as a shorthand for strlen(val). Likewise the various convenience
wrappers around it also accept -1. This was not documented, leaving
developers to wonder why len is a gssize, instead of gsize.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
8 jobs for gstring-docs in 24 minutes and 17 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #212501
cross-android_api21_arm64

00:06:30

passed #212502
cross-android_api28_arm64

00:06:46

passed #212503
cross-mingw64

00:07:25

passed #212500
fedora-x86_64

00:12:21

passed #212504
win32
msys2-mingw32

00:08:14

passed #213221
win32
vs2017-x64

00:07:48

failed #212505
win32
vs2017-x64

00:00:58

 
  Coverage
passed #212506
coverage

00:04:05

74.4%