Ensure all variables have a proper description if users are expected to ever use them

Currently we have in salsa-ci.yml a section with all variables:

variables:
  ...
  SALSA_CI_LINTIAN_FAIL_WARNING: ""
  SALSA_CI_LINTIAN_SHOW_OVERRIDES:
    description: "To make Lintian show overridden tags, set this to 1"
    value: 0
  SALSA_CI_LINTIAN_ARGS:
    description: "Arguments appended to lintian's generated arguments"
    value: ''
  SALSA_CI_PIUPARTS_ARGS: ''
  SALSA_CI_PIUPARTS_PRE_INSTALL_SCRIPT: ''
  SALSA_CI_PIUPARTS_POST_INSTALL_SCRIPT: ''
  SALSA_CI_DPKG_BUILDPACKAGE_ARGS: ''
  SALSA_CI_GBP_BUILDPACKAGE_ARGS: ''
  DOCKER_TLS_CERTDIR: ""
  SALSA_CI_DISABLE_APTLY: 1
  # These three ARM-related build jobs are disabled by default while there
  # isn't an ARM shared runner available
  SALSA_CI_DISABLE_BUILD_PACKAGE_ARMEL: 1
  SALSA_CI_DISABLE_BUILD_PACKAGE_ARMHF: 1
  SALSA_CI_DISABLE_BUILD_PACKAGE_ARM64: 1
  # And likewise for the RISC-V build job:
  SALSA_CI_DISABLE_BUILD_PACKAGE_RISCV64: 1
  SALSA_CI_DISABLE_BUILD_PACKAGE_PROFILES: 1
  # Disable cross-builds by default due to high failure rate that is not
  # suitable for CI that is expected to be green and gatekeep commits
  SALSA_CI_DISABLE_CROSSBUILD_ARM64:
    description: "Set this to 0 to enable the test that cross-builds the package (read more at crossqa.debian.net)"
    value: 1
  SALSA_CI_DISABLE_BUILD_PACKAGE_TWICE:
    description: "Set this to 0 to enable the test that builds the package twice in a row"
    value: 1
  ...

Only the ones that have a description or default value show up as prepopulated options on manual pipeline runs:

We should make sure that each variable we expect users to ever use, should have a description and default value. Many already have inline comments that could be turned into descriptions instead.

Additionally we should make sure that each variable that users are expected to use is also documented in the README.md. At the time of writing this issue, e.g. SALSA_CI_DISABLE_MISSING_BREAKS was not mentioned in the README.md at all.