Fix #14475 (Document unknownMacro) (danmar#8194)

Co-authored-by: chrchr-github <78114321+chrchr-github@users.noreply.github.com>

Author: danmar

man/checkers/unknownMacro.md

@@ -0,0 +1,68 @@
+
+# unknownMacro
+
+**Message**: There is an unknown macro here somewhere. Configuration is required. If AAA is a macro then please configure it. [unknownMacro]
+<br/>
+**Category**: Configuration<br/>
+**Severity**: Error<br/>
+**Language**: C and C++
+
+## Description
+
+Cppcheck has found code that is confusing and does not know how to analyze it. This is a critical
+error, the analysis of the whole translation unit is aborted. Such error in a header file can mean
+that analysis of many source files are aborted.
+
+Your code is probably OK but you need to configure Cppcheck to make Cppcheck understand the code
+better.
+
+## How to fix
+
+Review the configuration.
+
+If Cppcheck warns about a macro that is defined in a 3rd party library, and there is a cfg file for
+that, then a `--library=` option may be a proper solution.
+
+If Cppcheck warns about a macro that is defined in a header that should be included, make sure that
+this header is included properly. Cppcheck must have the include path.
+
+If Cppcheck warns about a compiler keyword add a `-D` that defines this keyword somehow. I.e. if
+cppcheck should just ignore the keyword then an `-DKEYWORD=` option is suggested.
+
+## Example
+
+### Example code 1
+```
+    fprintf(stderr, "Generating up to " F_U64 " sequences and up to " F_U64 " bases.\n", nSeqs, nBases);
+```
+
+Warning:
+
+canu-2.2/src/seqrequester/src/seqrequester/generate.H:72:41: error: There is an unknown macro here somewhere. Configuration is required. If F_U64 is a macro then please configure it. [unknownMacro]
+
+Fix:
+
+Somehow `F_U64` must be specified for Cppcheck to be able to analyse this properly. Either:
+ * Add `-DF_U64="x"` to explicitly tell Cppcheck what it should replace F_U64 with. Or;
+ * Add `-I..` so that headers are included properly.
+ * If the symbol is defined in a 3rd party library adding a corresponding `--library=` might solve such issue.
+
+### Example code 2
+```
+BOTAN_FUNC_ISA("crypto")
+void AES_128::hw_aes_encrypt_n(const uint8_t in[], uint8_t out[], size_t blocks) const
+```
+
+Warning:
+
+botan-2.19.5+dfsg/src/lib/block/aes/aes_power8/aes_power8.cpp:103:1: error: There is an unknown macro here somewhere. Configuration is required. If BOTAN_FUNC_ISA is a macro then please configure it. [unknownMacro]
+
+Fix:
+
+Somehow `BOTAN_FUNC_ISA` must be specified for Cppcheck to be able to analyse this properly. Either:
+ * Add `-DBOTAN_FUNC_ISA(X)=` to explicitly tell Cppcheck that BOTAN_FUNC_ISA("crypto") should be ignored. Or;
+ * Add `-I..` so that headers are included properly.
+ * If the symbol is defined in a 3rd party library adding a corresponding `--library=` might solve such issue.
+
+
+