Merge branch 'jc/environ-docs' into maint-2.38

Documentation on various Boolean GIT_* environment variables have been clarified. * jc/environ-docs: environ: GIT_INDEX_VERSION affects not just a new repository environ: simplify description of GIT_INDEX_FILE environ: GIT_FLUSH should be made a usual Boolean environ: explain Boolean environment variables environ: document GIT_SSL_NO_VERIFY