variable_watch: Remove undocumented and redundant access type

The ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS access type was switched on an
undocumented variable and its lookup caused an unnecessary performance
impact.  Remove it.
tests: test CMP0053 in WARN mode when watching variables

When CMP0053 is in WARN mode, variables get expanded twice, leaking the
fact that the string was expanded twice and changing behavior. Instead,
suppress variable watches when running the expansion to trigger the
CMP0053 warning.
Features: Add missing variable reference in docs.
Tests: Try to make CTestTestTimeout more robust

Write to the timeout test log file before sleeping and flush to be sure
it is created.  Move the check that the after-sleep line is not written
out to the ctest script.  Rename the CheckChild test to TestSleep since
it no longer checks.  Do not try to read the log file if it does not
exist.
Help: describe CTEST_BUILD_FLAGS
Help: Format and revise file() command documentation

Organize the documentation by sub-command to keep the signatures and
their descriptions nearby.  Use inline and explicit reST markup.  Revise
wording as necessary for the updated layout.  Clarify behavior of the
file(GENERATE) command w.r.t. conflicting file names.
Help: Add a manual for compiler feature control.

Link to it from the documentation of related properties, variables
and commands.

Extend the cmake-developer(7) documentation with notes on
extending feature support for compilers.
Merge topic 'variable_watch-no-allowed-access'

d8498003 variable_watch: Remove undocumented and redundant access type
Merge topic 'dev/CMP0053-variable_watch'

9ba91463 tests: test CMP0053 in WARN mode when watching variables
Merge topic 'compile-features-manual'

ca5d990f Help: Add a manual for compiler feature control.
b4d33f64 Features: Add missing variable reference in docs.
Merge topic 'revise-CTestTestTimeout'

9afcecaf Tests: Try to make CTestTestTimeout more robust
Merge topic 'doc-CTEST_BUILD_FLAGS'

b8d6ab60 Help: describe CTEST_BUILD_FLAGS
Merge topic 'doc-file-command'

d74ed543 Help: Format and revise file() command documentation