Better document purpose of BIOCONDUCTOR_ONLINE_VERSION_DIAGNOSIS

[mtmorgan]

• see Bioc-devel mailing list thread at https://stat.ethz.ch/pipermail/bioc-devel/2023-May/019678.html
• Purpose is to enable off-line use of BiocManager, including enforcing appropriate version of bioconductor for version of R, not to circumvent version check
• Relevant changes are to

  BiocManager/man/install.Rd

  Lines 85 to 95 in 187a7b5

  	\env{BIOCONDUCTOR_ONLINE_VERSION_DIAGNOSIS} is an environment
  	variable or global \code{options()} which, when set to \code{FALSE}, avoids
  	the R and \emph{Bioconductor} version checks that are done by querying
  	an online configuration file. Setting
  	\env{BIOCONDUCTOR_ONLINE_VERSION_DIAGNOSIS} to \code{FALSE} can speed
  	package loading when internet access is slow or non-existent, but
  	may result in out-of-date information about the current release and
  	development versions of \emph{Bioconductor}. Offline users should
  	set the \env{BIOCONDUCTOR_CONFIG_FILE} environment variable or option
  	to a \code{.yaml} file similar to \url{https://bioconductor.org/config.yaml}
  	for full offline use and version validation.

  (man page) and

  BiocManager/vignettes/BiocManager.Rmd

  Line 273 in 187a7b5

  ## Offline use

  (vignette)